Hi all, i am on the look for some great game to play co-operative on the PS3!

Please help me to find the game that's most fun with friends! Don't waste time playing poor games!

LittleBigPlanet 2?

Resident Evil 5 is good for bro-op, and The Mercenaries mode in it is a good party game. Castle Crashers and Scott Pilgrim vs The World: The Game are both genuinely fun and funny, and get better the more players you have. 

for 2 people army of 2 is alright and probaly dirt cheep right now

*edit* never mind just looked it up its still like £15 HOW HAS THIS RETAINED ANY VALUE

for 2 people army of 2 is alright and probaly dirt cheep right now

Online?  The Uncharted 2 maybe 3 one day.

RE5 has some of my favorite co-op gameplay in a long time. Killzone 3, LittleBigPlanet 1/2 and Uncharted 2 were all great as well.

Resistance: Fall of Man had surprisingly good coop campaign.

I can't believe Portal 2 has not been mentioned yet

Edit: I also have to agree that RE5 has some terrific co-op. One of the best times I've had co-oping

I can't believe Portal 2 has not been mentioned yet

@valrog: I guess it's true, great minds do think alike!

I thought Dead Nation was pretty good in local co-op.

I'm going to take extreme liberty with the "co-op" criteria and suggest Zen Pinball.

Pixeljunk Shooter 1/2

Resistance 2 (although it is admittedly a poor game)

Borderlands (another poor game imo, but other people swear by it)

Army of two (1+2) Are pretty good fun on co-op, me and my mate really enjoyed them, good emphasis on teamwork. Not great playing alone though.

Borderlands, Resident Evil 5, Portal  2, and Resistance Fall of Man all have pretty good co-op offerings. I guess Resistance 2 does, but I'm talking from experience on my brother's PS3 and I've personally never been able to round up 8 people to play it with. 
 
Also, Modern Warfare 2. Wait, what? It's got a mode called "Spec-Ops", which is a bunch of short little co-op missions. They're short enough to be in no danger of boredom, numerous enough to last almost as long as (or longer than) the short campaign, and varied enough to never get tedious. They are, however, balls-to-the-wall hard on the later levels and higher difficulties, which you'll need to do if you want to unlock all of them.  
 
EDIT: Also, Uncharted 2 if you don't mind playing online with your buddies instead of having them next to you. OK, I've never played it online, but my brother and plenty of other people have a blast with it.

For 2 people, couch co-op, I can recommend Plants vs Zombies. It has co-op adventure mode and co-op specific levels. I played through all of it with my girlfriend and it was the first game she had a lot of fun with.

Gibs

Little Big Planet 2, Renegade Ops, Fat Princess, PvZ, Castle Crashers, Worms, Dead Nation.