I'd recommend Ratchet and Clank, Ratchet and Clank: Going Commando, Ratchet and Clank: Up Your Arsenal, and maybe Ratchet: Deadlocked (it's more of a shooter than the previous games). I've heard the Jak games are fantastic, but I haven't tried them...
I'd also recommend Spashdown: Rides Gone Wild if you can find it because it was a pretty goofy/fun racing game.
If you like Lord of the Rings and RPGs, The Third Age (by EA) is actually a really good game, except the final boss which will leave you scratching your head.
Star Wars: Battlefront I and II are great. I know they're multiplat, but I've had tons of fun with them... Though I found that I prefer the first game because it has way better maps. Still, the second one has all those nifty awards and whatnot, plus you can play as Jedi.