Six drops have an incredibly high bar in cube, much more so than any other slot in the curve. Past six and you can only expect to be cast in a deck with loads of ramp or loads and loads of stall. This makes the distinction between cost of things 7+ pretty muddied and often fairly minor. Most 7+ mana cards have specific roles or synergies that allow you to use them more effectively. 7+ drops are things your whole deck is often tailored to support. Six drops are much more viable as stand alone cards and you can include them in a wider selection of decks than 7+ cards. Despite this there is still very limited space in the cube for six drops. From 3 CMC upwards competition for slots gets pretty tight, loads of good cards in that range and above are not main cube worthy. Of those more expensive cards that are you still commonly see them cut from lists because they have too many others.
A cube six drop needs to be very well rounded so as to go in a broad range of decks as well as extremely powerful so as to make them worth playing compared to a five drop or even lower CMC cards. There are a whole lot of six drops to chose from, there are even lots that look powerful enough for the cube yet there are very few that really shine in cube. Even this top 8 lists starts to tail off in power and amount played towards then end more than most lists I do. Before jumping in there are a selection of somewhat important honourable mentions. Some needed taking off the list because they were not true six drops. The defining thing about six drops is that they cost six mana! Those six drops that don't cost six mana for the most part do not therefor share the same troubles and trials as conventional six drops. Tasigur, the Golden Fang is my first honourable mention as he is a lot more of a one mana card than a six mana card. He is usually getting cast turn four or so as well. Great card but completely incomparable to those on this list. Real six drops laugh at a vanilla 4/5 with a 4 mana activated card draw effect. Tasigur doesn't even look good next to most 4 drops in cube but in just power per cost and playablity terms he would make this list of cards.
Next up on the honourable mentions is a card that goes the other way to Tasigur. Aetherling is six mana to cast but only the mad and the desperate cast it as soon as they hit six mana. Sensible people wait till 7, 8 if they have the luxury of time. Aetherling is one of the best finishers in all of magic, it is quite an oppressive card and it would be well up at the top of this list if I didn't consider it to practically be a 7 drop.
Finally there are two cards that didn't make this list entirely for being gold. Both Dragonlord Dromoka and Dragonlord Silumgar are incredibly powerful game breaking cards. Silumgar is a devastating and versatile swing ontop of a solid body. Dromoka is one of the best single bodies in the game for stabilizing a board, she is a massive threat and she is unusually good against both aggro and control. While both cards are super strong the issue a lot of six drops have is seeing enough play due to being so far up the curve. Being gold inherently makes a card narrow as does being expensive and as such the doubly narrow Dragonlords simply don't get enough play despite their power to deserve space on this list. Silvos, Visara, Rorix amd Duplicant would have all been at the top of this list when I made my cube. Cards like Oona, Queen of the Fae and Broodmate Dragon would have been high entries at the time of their release. These once staples are those that have been hit hardest by the power creep and basically never show up in cube decks (with the exception of Duplicant who still gets a bit of love here and there). Right, time for the actual list.
8. Noxious Gearhulk
One of the surprise performers from Kaladesh that has snuck onto this top 8. I didn't like the card much but it turns out it is incredibly playable. The body isn't great but it is still very relevant. It hits hard, is a pain to block and should trade with or outright kill most other stuff. While the body is nice it is the effect which makes this Gearhulk good. Being able to kill their best dork and get a nice big pile of life in return on top of the decent body is a massive tempo swing and a great way to stabilize a game. Being an artifact and a creature makes this powerful effect more more easily tutorable, repeatable and generally easily abused. This is much more heavily weighted on the answer side of things than it is on the threat side of things but it does still very much do both. The card has the feel of a Thragtusk when you play it except it scales far better against slower heavier decks. Being able to stabilize and disrupt at the same time is typically more useful than the one sided style of Thragtusk when you most need the card.
7. Linvala the Preserver
Not the most exciting card, in a lot of ways this is another Thragtusk style card. In many other ways Linvala is what you play when you don't have Elspeth Sun's Champion at your disposal which does mean this new Linvala is still a little under the radar. She does everything you want from a midrange or control top end card. She is one of the best board stabilizers as she creates a pair of large flying bodies that block well complete with a top up on life. She is also a pretty solid threat able to hit quite hard in the air and race well. You can leave one of the fliers back to defend while you get in for a bit each turn for a slower but safer clock. There is a bit of a theme within these top end dorks for threat that also stabilize well but Linvala is the most obviously designed that way. Her greatest asset is also the thing holding her back. She might be the best six drop you can make when behind and recover a game but she is pretty weak at sealing the deal when you are ahead or even. The really good six drops should threaten to end close games without the ideal answer right away and that isn't what Linvala does.
6. Torrential Gearhulk
Pretty early days for this one although I really don't see it going anywhere anytime soon. This has performed super well in cube thus far. Every time it is cast it offers devastating tempo swings and great value. A whole range of spells from top end Dig Through Time and Wretched Confluence to much calmer cheap cards like Counterspell have been replayed with this fat Snapcaster mage and it always feels really good for the Gearhulk player. You need some support for this card but you don't need the most abusive cards and you don't need that much. As a late play you will likely have a couple of targets for the Gearhulk to recast. A 5/6 flash does a lot of work on the board and combined with a free spell somewhat of your choosing seems to always lead to a complete blowout. There is a fairly high chance you can use it for removal, counterspells or card draw as the situation commands and that level of flexibility on what is already a good card pushes it really well. It has impressive stats for a blue card and the flash means you are that much more able to get immediate value from the body.
5. Consecrated Sphinx
I have always felt this was the blue Titan in spirit. The blue thing should fly and draw cards right? Sphinx would certainly be better if you drew two cards per attack or entering play a la Titans but it isn't that far off at all as it stands. There is a very small window in which you can stop it yielding value and after that the Sphinx doesn't need to attack and you have to avoid drawing extra yourself else it outperforms the Titan triggers. A 4/6 flier is highly robust and gives good board control. It is not the quickest threat but when you are drawing at least three cards a turn time is entirely on your side! Despite lower power the physical body of the Sphinx is more relevant than any of the Titans simply because flying is such big game. This is fortunate as drawing cards is not a tempo boost while all the real Titan effects do offer tempo. Sphinx is actually more popular in midrange decks than it is in control as getting it too late in the day as a control player really negates the value of the draw. The control player will typically employ safer and more specific draw tools and more reliable pure threats.
4. Inferno Titan
One of the less cast Titans in the cube but very much one of the most game ending. This typically hits harder than any other Titan and is harder to stop it doing that damage short of direct removal. Eating a pair of Arc Lightning before you get to try your hand at blocking this 6/6 typically leaves you with poor options at best. When you have no blockers this is doing at least 9, potentially up in the 15 area if you dump all your red mana into pumping it. Inferno Titan is almost always a big tempo swing when you make it but it is also an effective disruption and removal card rather than purely improving your side of the board. Aggressive red decks don't go to six drops and red midrange and control red decks are somewhat uncommon which is why Inferno Titan is not so commonly cast. That in turn is the only reason that it is the 3rd best Titan on this list and not higher. For pure effect this Titan puts in a lot of useful work very quickly on top of being a terrifying threat.
3. Grave Titan
For six mana you get 10/10 worth of stats over 3 bodies. You also get the ongoing threat of another 4/4 of stats over two bodies with each attack. Compare that to any other one man army card and Grave Titan blows them out of the water for stat efficiency. Like most one man army cards it is very hard to go round the Grave Titan. It can kill quite effectively while putting up more defeneces as well so for a card without lifelink or vigilance it is one of the hardest out there to race. Grave Titan is rare in that it is both the lead dork in a one man army and a card that generates value when it attacks yet it is huge and fairly well able to attack into most things. Precinct Captain is shut off quickly, Angel of Invention is picked off as soon as possible. Grave Titan is far more robust and consequently that much harder to stop it yielding value. The Titan is so powerful that it is one of the few cards as "cheap" as six mana that is regularly cheated into play with Reanimate, Sneak Attack and the likes. It has the feel of a lot of the super heavy seven and eight mana cards that you really can't hope to hard cast without warping your deck to do so. Grave Titan was a very strong contender for the top slot on this list. It is mildly less playable than the ones above it mostly due to colour reasons. Black is better than a lot of colours at housing a six drop but it is still a difficult thing to pull off well. The best Grave Titan's seem to be in blue or green decks!
2. Wurmcoil Engine
Somewhat of a big daddy Thragtusk that anyone can play. Indeed being an artifact often means anyone can play it from turn one onwards with Tinkers and Welders and that sort of cheating on the go. All colourless makes it very easy to ramp into as well. Just an all round great speedbump come threat. Near impossible to race given it threatens a 12 point life swing. Very hard to trade into well as you will still eat 9 points of lifegain and have to chew through 12/12 worth of stats on top of that. Either you need to exile the Wurmcoil or end the game now by going round it. Failing that it will either offer too much tempo or too much value for most creature strategies to cope with it. Wurmcoil has always been considered an honorary Titan by people I play with. Mostly this is just because it is the one six drop dork that is of comparable power level to the Titans. It is a good stand alone card you lay into most boards and be happy with the results. Most decks can play it and a lot of decks have a hard time against it.
1. Primeval Titan
I spent a long time trying to work out which was the best of the Titans and fell with Primeval for several reasons. Being green where ramp is fairly typical a six drop is much more like a five drop in other colours and is generally pretty playable in most decks. Green also has very few other quality six drops and so when you specifically want a six drop for your curve the Primeval gets a lot of priority. Most of the other Titans can be dealt with using the correct card. A Grave Titan can be Wrathed away, a Wurmcoil Engine can be Pathed. Primeval Titan gets two land that you cannot undo the value from in any reliably good way while dealing with the Titan. Cataclysm, Upheaval or Balance I guess are the best hopes but these are situational and build around cards. The first of which likely never works well for you against a Titan! Lands may be more robust than tokens or when they die triggers and they may be more permanent than some damage or life that you might acquire from other dorks on this list but lands after you have six mana isn't exciting right? In cube it turns out that is nonsense, lands are some of the best threats and value in the game. Colonnade? Wolf Run? It is incredibly easy to have a deck where Titan has two triggers (4+) worth of value lands to find in a cube deck. Not only do these lands help you win themselves they also let you do a whole lot more in your turns, more mana is more power after all. It might not add as much utility as the first two lands you got did in terms of casting things but they still add to your options and potency for the most part. You also get to thin your deck of lands helping you to draw gas should the Titan not close the game. As a minor aside, Trample is one of the better things to have on a 6/6 body and makes Primeval decent at attacking into chaff and being a relevant threat rather than just a source of value and board presence.
The makeup of a good six drop is pretty well outlined by this list. The ability to have a large and immediate impact on the game is key. Often this is in the form of stabilizing, both by adding bodies to the board, offering lifegain options and perhaps removing some of their stuff. Equally often the impact is in the form of value, extra draw on top of bodies or removal. Lastly the cards are typically quite robust. Those that can easily be dealt with by a single card tend to require a specific one to deal with them effectively. A lot of them will leave you with residual value when more conventional answers are used on them. These three things, stabilizing, value and reliability all seem more important than raw power in six drops. Certainly those that stand out most are the ones with these attributes as well as a very high power level!
I didn't want to honorary mention Frost or Sun Titan at the start of the list as I didn't want to give spoilers about them not being on the list. Both cards spent a long time in my cube and are certainly very high power cards that are cube worthy. The issue with both is that white and blue have more suitable alternatives to take the few slots that exist in my cube for six drops. Sun Titan is the weakest of the Titans. It is very poor at closing out a game and surprisingly situational in what it will do for you. Sun Titan also has diminishing returns on its effect as you run out of stuff to return. Closing a game with 3 or less CMC cards is hard at the best of times and you find yourself warping your deck to improve the value of your Titan. Sun Titan isn't a particularly good threat and its effect doesn't add to it enough to compensate. There are certainly decks where Sun Titan performs well but the typical cube deck can't get much out of him, the odd sac land is the norm after you get back your one OK 3 drop dork...
Frost Titan is a lot better than Sun Titan. The protection effect and tap down effect make him somewhat more of a threat. He can actually get in relevant attacks pretty well and is a bit of pain to take down. Frost Titan very nearly does all the things you want but it too often comes up short. Against a lot of dorks he can't be effective and is just a 6/6. A tap effect can lock someone out of a game but that is quite rare, it doesn't afford any card advantage whatever it is doing and so Frost Titan can fail to get you ahead even if it lives a long time. Tamiyo is a much better disruption tool and source of value as is Torrential Gearhulk. Consecrated Sphinx is a much better threat and source of value and nothing really compares to Aetherling when it comes to pure threats. Wurmcoil is the big stabilizer of choice for blue, failing that it is Ugin or Upheaval effects. Frost Titan looks like he does a bit off all the things you want but in practice he often fails to do any of them.