Burnout paradise, I had alittle more that 1000 hours on it. I did all 500 challenges about 6 times helping others. I still play this time to time. Game was a blast and still has a live community. In my opinion one of the best PS3 games to date.
MW3 for my online game. Over 90 days played and I've been to 20th about 4 times :p
As for offline goes I'd say GTA: SA. Had it on my PS2 and bought it on my PS3. Not sure if that counts but I've spent sooo much time on that game
medal of honor 2010 more tan 50 days, have the best respawn, have 6 killstreak (whebn you get the nuke you can suicide and killstrike count), 12 vs 12 al servers..... its the best game ever