I don't think I can nominate anything other than Phage, the Untouchable. Finally, a black creature that can single-handedly win the game that you CAN'T re-animate. You are encouraged to pay for the normal casting cost without any funny business. Plus, she's a fatty. What's not to love!?
Or, we could just go with one of the biggest and most unplayed critter in the game....Krosan Cloudscraper. This is one of the first beat-downs that I have nominated, mainly because it is not my style of play, but in a casual deck, this could get nasty....
This is my least favorite set of all time. But it has a handful of good cards. Most of them aren't too incredibly novel or fun. There's stuff like Akroma or Withered Wretch. Good cards, but probably not casual card hall of fame cards...
I would second the nomination for Phage if it mattered , but I'll nominate Scion of Darkness. He can be easily gotten out with three clerics and Dark Supplicant, is great in MP since he can steal creatures from any graveyard (once you attack), and it's one of the few prereleases I went to and thus have the prerelease card for it
Nominations so far:
Phage the Untouchable - Ransac Krosan Cloudscraper - DarthFerret Mistform Ultimus - Oversoul Clickslither - Mooseman Wirewood Hivemaster - Melkor Scion of Darkness - Spiderman