While I cannot attest to Persona 4, having never played it myself, I'd second the notion of picking up the others. Especially Ratchet&Clank; they are undoubtedly my favorite game series. Shout-out to Okami, Kingdom Hearts, and God of War as well.XIGBARx13 said:
To contribute something of my own (rather than merely flapping my gums in a long-winded "This!" post), I'd also suggest the Jak&Daxter and Sly Cooper series(s?) as well. The "Playstation Move Heroes" PS3 game combines R&C, J&D, and SC into a match made in heaven...if it weren't exclusive to the Playstation Move, that I have no other reason to want (grumbles to himself...).
And if you like those three, the developers did branch into some new IPs on the PS3, namely an alternate-history-with-aliens FPS (Insomnaic; R&C --> Resistance), an Indiana Jones-style adventure (Naughty Dog; J&D --> Uncharted), and an electricity-based superhero sandbox (Sucker Punch; SC --> Infamous). Just something to keep in mind for the future
Edit: Now that I think harder on the subject, I can think of so many great PS2 games: Burnout 3, FFX, the Katamari series (if you can handle zany Japanese-weirdness), the Prince of Persia games, Star Wars Battlefront II (if you had a friend who'd play with you); there's too many to count! lol