I'd have to say Bioshock, but Halo 3 was my most played and is definitely a very close favorite as well. So many hours in just custom games alone. Those were the good old days of Halo. When you could go on your recent players list and find any random playing custom games and join freely and they'd have no problem with it. The more the merrier. As I said, the good old days.
Mass Effect, S.T.A.L.K.E.R.: SoC, TES IV: Oblivion, and The Witcher being other favorites of that year.
Halo 3 is something that wouldn't be my favorite now, but man was it consuming my online time in 2007 along with Rock Band.
Funny Halo 3 story:
My manager (at the time) and I were playing online with a bunch of his friends, including this guy from the UK, all in party chat. It had been MONTHS since I had played Halo 3, so I was sucking wind, and his British friend was a smartass. We go into a 2v2v2v2 game, and the two of us get paired up.
So we're playing, I'm getting killed a lot, but he's kicking ass. At some point, we're split up, I'm searching for weapons, and I hear him shouting, "Shit, I'm being double-teamed!"
So I went, "Don't worry! I'm coming!"
And he goes, "Oh? Oh, you're coming? Well don't I feel so much better now that you're coming to save me?"
Never was owned like that before and I haven't been owned as badly since. xD~~