Halo 3, because my friend I always played games on the Xbox 360 with got it. I sold it again about 3 months later haha...
Otherwise, Battlefield 2142, which my friends decided to STOP playing soon after I got it. It's terrible compared to BF2, so that was a waste.
And I may be getting roped into finally purchasing CS:S. But I am resistant to the other rubbish spewing out of the broken Valve, like L4D and TF2.