Which PS3 games are still active/fun online?

Thinking of buying a PS3 since I have a gaming PC to play 360/PC games. My only question is which games are still good online for me and a friend to play co-op or competitive. He's really impatient so if there's an impossible learning curve please say so.

Games I'm curious about: MGS4, LBP, Uncharted 2, Killzone 2, and Resistance 2.

Any other multiplayer/co-op game suggestions are welcome as these are just off the top of my head.

P.S. Does Demon's Souls have good co-op?

Killzone 2, Modern Warfare 2, Uncharted 2, GTA Episodes from Liberty City. GTA 4 is kinda dead.

Those are my favorites.