Ace Combat: Zero has the best storyline, but Ace Combat 5 has the better wingmen AI (They can actually do Air to Ground attacks!). Very good games in their own rights though.
Star Wars Battlefront II, it is nigh impossible to find a copy of SWBI that isn't scratched badly enough to affect the game (Mine came with a scratch, and now I hear wookie calls in the stead of background music.
)
Katamari Damacy has probably THE best soundtrack of any Waaay-Out-There game, and is pretty cool to play too.
The .Hack//GU series is a RPG series, with a fairly silly game-mechanic of "Pseudo-MMORPG in a Playstation 2 game," but if you can get around the "psuedo game in a game" idea and the fact that all you really need to do is alternate between X and R1 to win, it is a fun, if very long game series.
Now, if you want to go into the "Essential Classics," on the PSN, they should have Megaman Legends 1 and 2.
G-Darius is easily one of the best arcade shooters ever, and was ported onto the PS2 in Taito Legends 2, though the PS1 version has the same, and comes with the case. Also available on the PSN.
Megaman X4 and X5 are also great games, and just as difficult as a Megaman game SHOULD be.
I have heard good things about the Ratchet & Clank series, same with the Jak & Daxter series.
On a final note, do NOT get Secret Weapons Over Normandy or IGPX. Not very good games.
