Lets see...the 7 cards I hate the most, hmmm....
White: Serra Avatar
Black: Leshrac's right (I play black and there's nothing worse than a Serra Avatar w/ swampwalk...
Red: Landslide, that last resort card has cost me more than one game...
Blue: Any form of counterspell
Green: Thorny/Rhox
Artifact: Masticore...those damn things really knock out my discard deck in T2 play...
Land: Gaea's Cradle, I can't get my hands on one!!!