I've only bought four so far:
Final Fantasy XIII
MAG
Battlefield: Bad Company 2
God of War III
Even though MAG has given me the most hours, my favourite would have to be FFXIII. I've finished the main story and now that I have the ability to roam and do whatever I like, I can't stop playing it. Good game that people didn't really give a chance.
Saying that though, I'll be getting Red Dead Redemption this week and it looks like that will be my new favourite...