I like RPG's myself, but unless they're you're thing you may not:
Shadow Hearts [Covenant, the sequal is even better, and doesn't require you to play the first one, uh, first]
Dark Cloud [and Dark Chronicle / Cloud 2]
Final Fantasy X
Megaman X: Command Mission
Kingdom Hearts [and Kingdom Hearts 2 - Don't be put off by the disneyness, they're great fun]
They're my crop of top ps2 games, thou I really disliked Final Fantasy X-2 so I gotta recommend staying away from it.