Another thing I failed to realize (well, remember) about Mirrodin block is that scry wasn't in Mirrodin itself and so we didn't get to see it until 2004. The arrival of scry cards is what pushes this year above all previous since 93 in terms of good cube playables. Scry is probably the best mechanic introduced into the game and making it an evergreen effect is one of the best moves Wizards have ever done. Scry makes everything more consistent and it helps out the whole deck but it is very hard to abuse at all and presents no threat. Although cycling is also a great mechanic that helps to improve consistency it only really helps the cards it is on while scry helps everything and that is why I rate is as the best.
I also thought Mirrodin was the combo year but again, most of the combo support cards come from Darksteel and Fifth Dawn as you can see below in my usual list of other good cards from the year.
Sensei's Divining Top
Azusa, Lost but Seeking
Boseiju, Who Shelters All
Crucible of Worlds
Glimpse of Nature
Heartbeat of Spring
Kiki-Jiki, Mirror Breaker
Lantern of Insight
Plunge into Darkness
Pulse of the Fields
Time of Need
Zo-Zu, the Punisher
Yosei, the Morning Star
10. Engineered Explosives
A highly versatile and playable little answer card. Great for taking out unusual permanent types and superb at killing off tokens. It is a more cost effective and more generally playable version of Pernicious Deed. While Explosives lacks the raw power and scaling of Deed it is much more about the early game and being one mana cheaper to take out a specific target is a big deal. Being able to take out planeswalkers is really nice too although charging Explosives upto four can be a challenge. Another aspect of Explosives playability is that it is specifically the CMC you blow up not everything upto it as well which means you can afford to play the card in a deck aiming to develop a board. Explosives has its potency a little bit linked to Trinket Mage. You don't need Trinket Mage to play Explosives or have them be good by any means but they are a really good pair, rather like Stoneforge Mystic and Batterskull. Being able to tutor up versatile answer cards is a pretty big deal! Sadly I have cut Trinket Mage due to Grey Ogres not being very good and a seemingly continual decline in both the range and the number of other good targets for Trinket Mage. Due to the present low value of Trinket Mage I have only rated the Explosives relatively low on the list. It is more of a stopgap for a problem than something you actively want.
9. Death Cloud
I think I claimed Upheaval was the last in the powerful effects balanced with symmetry but it isn't, this is! The key difference is that Death Cloud is actually quite a well designed, fair card. It is hard to use oppressively and so I don't consider it one of those bad polar cards like Upheaval and Armageddon. Certainly the Cloud can be oppressive and it will straight up end the game if you get to land a good one but it is not something you can just throw in some mana rocks and have reliably do what you want like Upheaval. Death Cloud is surprisingly versatile and feels kind of like a charm or command style card. Generally the outcome is that you pay some of your resources so as to hit some of your opponents other resources a little harder. Perhaps you get to Wrath them but you have to lose your hand to do so etc. Cloud is one of my all time favourite cards although I am learning that a lot of my favourite cards seem to come from this era (Lava Dart, Chrome Mox, Cabal Therapy etc) and so I think I am just biased as this is the era I was playing the most magic by a long way. I was playing every format that had sanctioned tournament support basically and so I expect the greater exposure and discovery aspect of the cards is a big part of my fondness for them. I would have though the '95 cards were my fond cards as that is when I began magic but it would appear not!
8. Sword of Fire and Ice
I have never been a huge fan of the Sword cycle, they have always felt like unnecessary risk taking. A well aimed removal spell and you are super behind with your big clunky equipment. As creatures have gotten more powerful the return on Swords has become less pronounced and with tempo being increasingly important the risk of Swords is rather increased. There are not many things that will put me in a position where I want to play a Sword. Lack of playables is the most common. Having only weak small dorks because I am black and/or blue is the other more common reasons. Sword is just a problem solving tool rather than a good card in its own right I would say these days. It is a nice rounded card that does direct and core things (cards and damage) and so it is a good fit in any place where equipment is viable. Fire and Ice is not just the most playable but probably also the most powerful of the cycle. While I don't greatly rate the Swords these days and have never much liked them it would be foolish to deny that back when they first came out that they were immensely powerful in a lot of formats. Sword of Fire and Ice was absolutely a fine first pick in cube, powered or otherwise and that likely remained true all the way to Zendikar block kind of era.
7. Isamaru, Hound of Konda
The one mana 2/2 with no drawback was a milestone mark in magic. I remember being very excited to play with such a potent new card. Isamaru has aged pretty well, indeed it likely just got a chunk better with Mox Amber! Isamaru is the oldest pure aggro dork I have in my cube. Power level wise Isamaru is still one of the better one drop beaters on offer however lack of human synergy is presently an issue leading to Isamaru being one of the least valuable one drop white dorks in the cube. Being so vanilla as well Isamaru doesn't engender interesting choices and quirky exotic games. Zurgo, Incited Rabble and the like are all way more interesting cards to play with and they both have similar function and power level.
6. Sakura-Tribe Elder
The second and last of the Champions cards on this list and a big part of why the other one never shone in constructed at the time! Tribe Elder is a fine all round card. It is a Rampant Growth which is a perfectly acceptable ramp and fixing tool but then it has a far from insignificant repetoire of extra uses. The most basic and common of which is simply denying one attacker damage for a turn. That alone would make Tribe Elder one of the best Rampant Growths on offer. Then you have those times you just leave the Tribe Elder in play and use it for raid, for revolt, to equip something to etc. Your opponent doesn't know if you are ramping with Elder or keeping it about until it is too late. Simply being able to hold off on making choices about your Seal of Rampant Growth is pretty good. Being a dork that gets lands is nice too and lets you abuse it with a wide selection of recursion cards. Tribe Elder defines green perfectly
5. Night's Whisper
One of the only pure draw spells left in cube due to being efficient and cheap enough to actually play. Night's Whisper is the second most efficient stand alone card advantage card in magic after the infamous Ancestral Recall. You don't need to set it up like Chart a Course or Predict or Treasure Cruise or Deep Analysis nor do you have to wait as per Ancestral Visions. Night's Whisper is just a nice cheap little way to pull ahead in value. It is cheap enough that the tempo concession is minimal and it is clean and unfettered sufficiently to be playable in most black decks. Something like a Dark Confidant is a higher risk card in many ways. High risk in that getting it killed results in no extra cards and not getting it killed might result in your taking too much damage. You might well also have to consider your build that much more carefully if using Confidant style things for your draw. I somewhat class Night's Whisper as cheap filler and will often use it to increase my low curve playables in a list.
Power Sink's finest form! The generally useless Mana Short aspect of Power Sink is traded for an epic scry 2. Given that Dissolve is scry 1 and 3 mana and fairly decent you are looking like getting good value out of Condescend in most cases. I have cast it for 0 on a couple of occasions simply for the scry and/or storm count. Even when you have to pay four to stop the thing you are generally pretty happy with the outcome and most casts are for 2 or 3 mana. Most of the time it is more it is no problem as you both are a little flooded. Certainly not a hard counter but harder than most, you feel safer with Condescend in your deck than you do with Mana Leak I would say. Condescend pretty much always gives you the option of spending no mana in a turn and being a hard counter, it is the option to have it be a soft counter on the turns you do do things that helps to make it so great. Mostly it is the scry 2 but still. Scry 2 is more than twice as good as scry 1 and just exactly what you want to be doing as a control player. Condescending a curve play on your turn two is about as good as it gets. Scry two done well that early on in concert with a good tempo 1 for 1 play is a huge leg up and probably the most powerful counterspell possible. Memory Lapse can punish a screwed play or a deck all about the curve and Remand on a curve play always feels great but I think Condescend takes the prize.
3. Serum Visions
Straight in with another core blue scry 2 effect. We all know how good these kinds of cards are and I am sure I have written at length about them with not much having changed in regard to such cards since then. Most cards that cycle for one mana are good even with limp effects and scry is far from a limp effect! While Visions isn't quite as choice as one mana card quality goes for constructed in cube it is premium. Without the same reliable access to sac lands the scry card quality cards perform better in most cases than things like Ponder and Brainstorm. The difference between Serum Visions and Preordain is negligible in cube too. Both are great, the latter is more immediate quality and is a better late game draw however Visions does offer some better synergy with top of library effects and so it rather balances out. They are hardly that different in power in constructed formats!
2. Eternal Witness
I remember reading this on the spoilers and assuming it was a fake or at least a copying error. Both the spoiling process and the internet were a lot less established and polished back in 2004 and spoilers were commonly unreliable, until you saw a real card you didn't trust things like you do now. I didn't trust Witness, it seemed far too good at the time to be something they would print. It was indeed a very good card at the time, it would not be at all unfair to call Eternal Witness the original Snapcaster Mage. It was easily as powerful then in standard and in cube as Snapcaster is now. Witness has become increasingly fair. The tempo is low and the value is only mild as a 2/1 isn't super exciting. Recursion is however super good in most formats and singleton ones cherish it dearly. Being a creature also allows for some nice loops you can setup.
A confirmed whoopsie by the design crew at the time. Something like a last minute change that was supposed to make it worse and ended up making it better (+1/+1 to +1/-1? I think). Clamp is a tempo loss card and as such it has lost value in the new meta where it is more important to have a tempo lead. That is not to say the card is weak, it is far from it. Nor is it slow and clunky. Despite not being a proactive card it is still too good, and it is quite hard for such cards to be all that oppressive. In any sort of even game a Skullclamp will lead to a win as you will just have never ending cards. It is like having a Yawgmoth's Bargain in play except you pay mana rather than life. You have to race a Skullclamp as you cannot out value it. One of the things I like about Clamp is that it is actually quite hard to use properly, to know when to use it to dig, when to use it for protection and when not to use it at all. It is beyond tempting to cash in all your dorks, as the sage wisdom of Mr Chapin advises; "drawing cards is life" and so to Clamp is to live. Sadly "living" in this capacity will oft lead to losing the game of magic, all be it with a very full hand! There is a strong argument for Skullclamp being the most broken card printed since the Urza's block era. It would be very interesting to see what formats with it unbanned in would look like. Certainly it doesn't dominate cube like it used to but cube is not modern etc.