As of this writing (8/14/13), the list which follows is terribly out-of-date. In addition to judging respective power levels of all the cardsI've obtained in the intervening years, I'm also considering nominating the best (or best few) cards that I own of each possible multi-colored combination. If you were brought to this site while searching for any of the cards listed herein, and were hoping for something more substantial, I do apologize.
The following is an ordered list of my best Magic: The Gathering cards (playability-wise) thus far, by color. For simplicity's sake, no Commons are included.
White: Armageddon (Fifth Edition, Rare); Akroma, Angel of Wrath (Legions, Rare); Parallax Wave (Nemesis, Rare); Serra Angel (Fourth Edition, Uncommon); Akroma's Vengeance (Planechase, Rare)
Blue: Evacuation (Stronghold, Rare); Equilibrium (Seventh Edition, Rare); Serra Sphinx (Planar Chaos, Rare); Azumi, Lady of Scrolls (Champions of Kamigawa, Rare); Fatespinner (Mirrodin, Rare)
Black: Braids, Cabal Minion (Odyssey, Rare); Infernal Tribute (Weatherlight, Rare); Hypnotic Specter (Fourth Edition, Uncommon); Grave Pact (Planechase, Rare); Sengir Vampire (Torment, Rare)
Red: Savage Beating (Darksteel, Rare); Relentless Assault (Eighth Edition, Rare); Shivan Dragon (Magic 2010, Rare); Flametongue Kavu (Planechase, Uncommon); Skullscorch (Torment, Rare)
Green: Sylvan Library (Fifth Edition, Rare); Gaea's Anthem (Planar Chaos, Rare); Azusa, Lost but Seeking (Champions of Kamigawa, Rare); Enshrined Memories (Betrayers of Kamigawa, Rare); Eternal Witness (Fifth Dawn, Uncommon)
Multi-colored/Gold: Shadowmage Infiltrator (Time Spiral, "Time-shifted" Rare); Void (Invasion, Rare); Eladamri's Call (Planeshift, Rare); Stormbind (Time Spiral, "Time-shifted" Rare); Fires of Yavimaya (Planechase, Uncommon)
Artifact: Mox Diamond (Stronghold, Rare); Umezawa's Jitte (Betrayers of Kamigawa, Rare); Cloud Key (Future Sight, Rare); Black Vise (Revised, Uncommon); Crucible of Worlds (Fifth Dawn, Rare)
Land: Lotus Vale (Weatherlight, Rare); Undiscovered Paradise (Visions, Rare); Gemstone Mine (Weatherlight, Uncommon); Sunken Ruins (Shadowmoor, Rare); Heart of Yavimaya (Alliances, Rare)