What's your favourite online game for the PS3. And yes it can be multiplatform for example Call of Duty.

Here are some good suggestion:

Warhawk

Call of Duty Black Ops

Call of Duty Modern Warfare 2

Call of duty Modern Warfare

Uncharted 2

Killzone 3

Killzone 2

Little Big Planet 2

Little Big Planet

Grand Theft Auto 4

Metal Gear Solid 4

Gran Turismo 5

You can pick any game as long as it's on PS3

Killzone 2

Killzone 2.

Resistance 2

I loved Modern Warfare 2 :)

Edit: Forgot about Uncharted 2 and 3 is going to even better. 

Metal Gear Solid 4, virtually tied with GT5 and Uncharted 2 but I'll stay with MGS4 as Numba Wan!

uncharted 2 and killzone 3

Wow no one liked games like Black Ops or Call of Duty 4, or even Warhawk but chose Killzone 2! That's pretty sad to me

killzone 3

Well the only game I've ever played on PSN is Killzone 3 so I have to say that even though I didn't really find it enjoyable.