I still play PS2 games on my PS3 occasionally. Just recently I played Radiata Stories, which is a Square Enix game, and it was a really fun game. 177 total characters that you can recruit (step aside Suikoden, still love you though) and the game had some catchy music from Noriyuki Iwadare. There wasn't a whole lot of story, but overall I liked it.
Some of my favorites are:
Silent Hill 2 and 3
Resident Evil 4 (although not technically an original PS2 game)
Red Faction
Star Ocean: Till the End of Time
Wild Arms 3 (one of the best rpg's on PS2)
Legaia 2 Duel Saga
Final Fantasy 10 and 12
Kingdom Hearts 1 and 2
Tales of the Abyss
Tales of Legendia
Castlevania Curse of Darkness
Suikoden V
And probably a few others I forgot.