I can't really decide on one speciffically so I'm goiong to break it down by colors.
Artifact: Draco, sure he costs 16, but he's still pretty cool, especially in a 5c domain deck.
Black: Devouring Strossus or Avatar of Woe, can't really decide.
Blue: Tidal Kraken, it's unblockable, nuff said
Gold: Dakkon Blackblade or Lord of Tresserhorn, Dakkon's one of my all time favorites and the Lord's a 10/4 regenerator.
Green: Thorn Elemental, it's basically pointless to block it, plus it just looks cool.
Red: Two-Headed Dragon, or Rathi Dragon, sure they're not that big, but they are cool, plus they fly.
White: Serra Avatar, it can get pretty small, but White's the color of life gain so it can get pretty big too, I once had two 432/432 Serra Avatar's out at the same time, that game was brutal.
Land: Mishra's Factory, the original man-land.