Getting a double kill tomahawking a RC-XD, and tomahawking another one in a different game (no kill though).
I'm not sure probably one of the following although I've beaten plenty of others as well. Beating ff7 with everyone in the party at level 99 and every item in the game Beating X-com Ufo Defense Beating Super Street Fighter 2 in the arcade on my first ever play without losing a round. Beating Ridge Racer Revolution (last race had to be pretty much perfect) Beating Morrowind and the two expansion packs I really don't know which one is harder or more impressive.
