Assassin's Creed II - loved number one, two improved it and streamlined it
Mass Effect 2 - pretty much the same argument as for ACII
Shadow of the Colossus - went in expecting nothing but a bunch of epic boss fights, got so much more
Bioshock - expected a game that could possibly rival HL2 for my favorite FPS, didn't beat it, but definately tied.
GTA IV - best GTA game according to me. Everything came together in a package that really worked for me (I know a lot of people don't think so)
Halo 3 ODST - expected more of the same, but ODST nailed something that the other halo-games didn't have, at least for me
Mercenaries 2 - basically Playground of Destruction with beefed up graphics. Only downside was no World in Flames-mode
Kingdom Hearts - sleeper hit. Really thought it was going to be ridculous, Disney mixed up with final fantasy... Really surprised me, now one of my favorite PS2-series
Kingdom Hearts II - Better combat, easier narrative to follow, and a difficulty that was more suitable for me (I got baldy pwned in the first game)