For me it's MW2 MP.
Seriously, on one hand it's an awe-inspiring tour-de-force with MP moments of sheer nail-biting adrenalised terror (getting nukes, beastly one-shot kills with the Intervention, resorting to picking up weapons to stay alive, bringing a Javelin down on Flag A at the start of a Domination match, taking out 4 bad guys with a shottie in one room)... and on the other it's a bucket of **** with akimbo, noob tubes, quick scoping, commando, knife runners being shot in the face and still killing you, hackers everywhere, constant disconnections and host migration to name but a few.
*pants heavily*