PS2 because there are only a handful of games that are actually designed from the ground up for online play.
Out of the single player game for this gens.. I'd say the PS3. I've owned a 360 btw and played all the best games for it, no bias opinions here.
The best single player PS3 games:
Metal Gear Solid 4
Uncharted: Drake's Fortune
Valkyria Chronicles
Other good single player PS3 games:
Ratchet and Clank Future: Tools of Destruction
Assassin's Creed
Ninja Gaiden Sigma
Killzone 2
Resistance 1 & 2
You shouldn't include multi platform games.