Tempest, Stronghold, Exodus
Tempest block continues the power level and card design evolution, and should definitely be checked. Shadow creatures are great for aggressive decks, and various other interesting cards appeared in this block. Tempest brought Aluren, Capsize, Corpse Dance, Intuition, Reanimate, just to name a few. Stronghold and Exodus also featured powerful and unique cards, like Carnophage, Ertai, Oath of Druids, Survival of the Fittest, Wall of Souls and Mox Diamond.
Urza's Saga, Urza's Legacy, Urza's Destiny
Urza block was a local maximum of card power. Even though the standard of that time was destroyed by some extremely powerful combo decks, the set's individual cards have great average quality, especially the blue ones. Some are quite unique such as Show and Tell, Greater Good and Smokestack, others are just staples such as Stroke of Genius, Powder Keg and Rancor.
Mercadian Masques, Nemesis, Prophecy
A huge drop in power level, with very few worthy cards that only appeared in this block. Cho-Manno, Revolutionary, Dust Bowl and Gush are some exceptions. The two smaller expansions are even worse, with practically only the Avatar cycle being decent.
Invasion, Planeshift, Apocalypse
Due to the multicolored theme, most cards, designed for a multicolored environment, probably won't work in a cube. The best ones are the good multicolored cards, such as Spiritmonger, Pernicious Deed, Mystic Snake, Fire/Ice, Void and Absorb. A few good green enablers for multicolor can be found, like Harrow, Quirion Elves and Fertile Ground.Other random good cards such as Flametongue Kavu and Fact of Fiction also exist. The rest is only good when playing 3 or more colors, or a specific combination of 2, so only multicolored focused cubes could profit from them.
Odyssey, Torment, Judgment
Odyssey is quite linear in its graveyard theme, so most cards simply won't fit a cube. Despite this, flashback is a non-linear mechanic, featuring great cards like Firebolt, Chainer's Edict and Moment's Peace. There are also good discard enablers to support reanimation and graveyard themes, such as Wild Mongrel and Compulsion. Finally, some random gems stand out, including Grim Lavamancer, Phantom Nishoba, Phantom Centaur and Faceless Butcher.
Onslaught, Legends, Scourge
The linear tribal theme makes these expansions not very remarkable for non-tribal cubes, but they contain some nice cards like the 3CCC legends (Visara, Arcanis, etc.), Ravenous Baloth, Siege-Gang Commander and Akroma. The overall creature power level starts improving in this block, as the mentioned cards show. If the cube will contain a good amount of morph, most good options here as well, such as Exalted Angel, Hystrodon and Bane of the Living.
Mirrodin, Darksteel, Fifth Dawn
Mirrodin is plentiful in good cards for the cube. Firstly, the high artifact count means there are several decent cards in a category that tends to be weak in cubes. Another factor is that equipments first appeared here, so there are plenty. Too bad the artifacts matter and affinity themes take up a good share of the cards in this set. Darksteel is even better than Mirrodin, featuring more modularity and lots of cool cards, such as Fangren Fistborn, Darksteel Colossus, the Pulses and Aether Vial. Fifth Dawn pushes too far into the prismatic theme, which won't fit most cubes - only those that support domain. There are, though, a few cards - mostly artifacts - that stand out, such as Eternal Witness, Wayfarer's Bauble, Engineered Explosives, Vedalken Shackles and Grafted Wargear.