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DBZ Budokai Tenkaichi 1 & 2 ( I still have Tenkaichi 2, but no ps2...)
Metal Gear Solid 3
NBA 2k5 & 6
Killzone (first game i ever played online)
Star Wars Battlefront 2 (simply amazing)
Personally, I'm partial to Tony Hawks Underground and the Original Lego Star Wars as well as stuff like Half-Life and Kingdom Hearts 1&2.
I'm also a fan of the guitar hero franchise, but only get the ones not avaliable on a newer platform, i.e. 1 and Rocks the 80's. In fact, just buy the first one.