0 - unplayable in 40 card singleton
1 - effectively unplayable
2 - has low tier constructed decks it might go in
3 - has mid tier constructed decks it does go in
4 - pretty powerful stuff with several potential homes, able to perform well in lower powered cubes
5 - powerful stuff that is either just too narrow or has too many superior alternatives
6 - fringe cube worthy
7 - cube worthy
8 - cube staple
9 - unpowered cube bomb
10 - powered cube bomb
Phyrexian Fleshgorger 5
Prototype is an interesting un-kicker effect. I suspect there will be some sneaky interactions with them, most likely those where you get to "cheat" on having a big CMC dork in play. Fundamentally kicker is a good mechanic and so prototype should follow suit with that. Modality is great on magic cards and even more so in limited formats.
Just because prototype is good doesn't mean this card is however. Much as I am loving ward as a mechanic at the moment too I find it hard to get excited by this. Seven is a lot and will not happen very often which means you really have to want a three mana 3/3 in your deck. Yes, it has bells and whistles but it isn't that big, threatening, or able to generate any sort of value. It is just a very mid level beater. I like the card and it isn't weak but it isn't very well suited to cube and doesn't excite me much at all in the way a Kavu Titan doesn't excite. I am at least looking forward to seeing all the other prototypes the set has to offer.
Urza, Powerstone Prodigy 2
Nice to see this as an uncommon without too much pomp. I like powerstones a lot and look forward to seeing a deck that uses them well. Obviously we all love a looter but sadly for young Urza the cube is not after these sorts of effects at three mana. Cards need more impact than this at three or more mana. The only place I could imagine playing this was a deck themed around powerstones in some way. I certainly like this a lot but power creep ensures we don't get much opportunity to play with fun cards like this any more if we want to be at all in with a chance of winning. Yes, yes, commander can play with this
Ashnod's Harvester 1
Too low power overall. The body is a very weak beater and the graveyard disruption effect is either very minor or too unreliable to be worth including. Play a threat or play graveyard disruption. Or play Graveyard Trespasser which is at least just a lot more powerful than this!
Mishra, Excavation Prodigy 1
Cute but fiddly and fragile. Mostly just too much mana up front to be something you really ever want. Obviously this would be a total bomb if you could trigger it more than once a turn so there is that...
Hurkyl, Master Wizard 4
The more things you try and draw with this the less consistently you will do so. Aiming to consistently get one card is likely the best strategy with using this. In my cube this is realistically just going to get instant and sorcery cards although when you build around this artifacts and enchantments enter the discussion. It is a fun card with some good high roll entertainment value. For cube it is a little fiddly and unreliable. If this were just draw a card at end of turn I would be a whole lot more interested. The cost is low enough and the stat line sufficiently bulky and defensive to all line up OK. A sort of blue Courser of Kruphix! But no, I think prolly don't be playing this unless you are very deep on the synergies.
Urza, Prince of Kroog 2
Tempered Steel on legs! Also a good mana sink for all those powerstone tokens we seem to be making! Certainly this isn't going in a cube without a heavy synergy setup but I am not even sure if I want this in many constructed decks either. Certainly if there is enough low end artifact beaters in standard this could be very naughty but in a singleton eternal deck? Can't see it, there are just more powerful payoffs for the artifact decks.
Mishra, Tamer of Mak Fawa 1
I like giving my stuff this ward very much. Sadly the unearth ability is narrow and fairly hard to make useful and the vanilla 4/4 body is very unimpressive. If this had a bit more presence or relevance while on the battlefield it would be most interesting but as it is this is a bit of a non-starter. You just can't go around paying five mana for such low threat and board presence. So rarely will this be able to attack safely and profitably. I would rather have a Carven Caryatid (2/5 defender for three with EtB draw a card) and that is not a card you see these days. Five is a suitably prohibitive cost in high powered environments like cube that you really need those cards to be proper game winners.
Argoth, Sanctum of Nature 2
Mostly this is coming in tapped and so it doesn't quite get there. Five mana to make a 2/2 is a long way off the tempo one needs to compete in cube. As far as utility lands and late game mana sinks go Lair of the Hydra and Kessig Wolf Run have this well and truly beaten. The mill is more likely a preventative thing than a boon given how late you are going to be using this. You want your self mill cheap so you can get it going early. This isn't a weak card but it is a way off for cube as a stand alone. Tribal bears is perhaps the only place I would play this if not planning a meld. Perhaps a commander deck using things like Life from the Loam and Crucible of Worlds too? There is no way you are then not playing Voice of Gaea then as well though. In conclusion, unless tribal bears this is only playable alongside Voice of Gaea, maybe standard too, I can't speak to that. Not my area.
Titania, Voice of Gaea 1
This is another card you are not playing by itself. Double up on that Zuran Orb! Woot. The body is fine but it isn't enough to be playing the card for, even with a smattering of random life gain added to the mix. Without Argoth this is just not packing the power needed.
Titania, Gaea Incarnate 6
Well obviously this is massive. It is flipping and melding into at least a 6/6, likely an 8/8 or 9/9 at the low end. It is also providing a huge mana influx along with fairly significant card advantage. It even has a mana sink and means to threaten the game without having to get Titania into combat. Most games Titania will win quickly by herself but those she does not she can win by animating all your lands. So yes, flipping and melding this is very big game. It is likely to win the game and is not too significant of a mana investment or cost to your deck. Argoth is likely only a fraction weaker than a forest and Voice of Gaea is a passable middling dork. You are likely only looking at an investment of 4 mana to get this all going with relatively minimal damage to your build. That being said you are not flipping this often without some effort, you want self mill and/or looting and lands you can easily put in the bin. You probably also want the means to tutor up the various parts of the meld, perhaps even with the means to protect it. I am imagining a deck with Grapple with the Past, Traverse the Ulvenwald, and perhaps Blossoming Defence. Without that kind of support you are just never realistically flipping this and thus there is little point including it. To me that makes this a build around and something you should only bother with in constructed formats. That being said, it is relatively painless and easy to include and support so perhaps it will be a relatively strong thing to do in those constructed settings. I certainly plan to include this in cube for a while just to get those achievements! Very likely to get cut from cube shortly after that first meld win however.
Splitting the Powerstone 1
Very narrow and seemingly not even all that powerful. I am not sure where I would want this. I feel like it needed to cost two to make it cube interesting but I imagine that could have made it a bit scary in some other formats. This just smacks of a support card but at three mana you are typically past support cards. This also rather needs to draw you a card to be of decent power level and that is going to be hard to pull off. A nice way to use extra legendary Mox up but you don't tend to have spare extra legendary cards in singleton formats! I do want to find a deck with cool uses for powerstone tokens but even then I suspect this card will be on the fringes.