Lately, it's been co-operative multiplayer: While I personally believe that humans are much more formidable opponents than any AI algorithm created, it's the sense of accomplishment you get when going through a co-op mission in ARMA 2 that's even better than having a kdr of 3:1 + the highest score (which means I wasn't just playing tdm, I was 'running the bomb' too) in an objective base multiplayer of CoD4 delicious sabotage.
That said, Modern Warfare 2 Spec-Ops, Borderlands, Left 4 Dead 2, ARMA 2 co-op, that's a lot for 2009 I think, and that's just awesome.