two of my favorites are Last Laugh and Desolation angel.
I once played a 6 man game, 2 of them using elves. I dropped Last Laugh for the first time, and everyone was wondering what the hell it could do. I then played a plague bearer...and ended the game right there. about 35 creatures total were out in play, and the 22 elves were the first to go, adding to the damage done to everything and everyone. That cleared all the rest of the creatures, and I won thanks to Congregate the turn before.
As for the angel...it's fun to drop with the kicker so no one can do anything more for most of the game.
Thanks guys, I tried to look it up on Urzas Hot Tub, but was unaware that it was all one word, so my attempt failed. Now i know what you were talkin about....but I still say the most fun card is Wheel of Fortune (Forked of course)