As far as online goes I'd say:
1. Socom: Seems to be a love it-hate it affair, If you're looking for a great online experience, this is a great place to look. There's a large following of people out there that really LOVE this game for good reason. Bundle comes with the official headset if you don't have one yet which alot better than the warhawk bundle.
2. Resistance 2 (co-op): The co op mode is very unique and you could put lots and lots of time into it if you wanted to. Competitive multiplayer is alot of more of the same Halo-esc deathmatch.
3. CallOfDuty (4 or Waw) This game fell down the ladder for me largely cause i played 4 a whole lot last year, and Waw was too similar, Has a lot to upgrade, but seems to lack some of the replayability the first two on my list have.
4. Warhawk: Good game with solid graphics and gameplay - it's really 'a lovechild between unreal tournament and battlefield' but third person. Not too expensive of a game either.
There's alot of games out there like little big planet that have online modes, but not as much of an 'online focus' like the games i listed above.
MGO: A real hassle to get started, really seems to me to be a good game to people that love Metal gear, but if you don't you won't think much of it. This is kind of a free bonus you get for buying metal gear solid 4 - I'd buy this game for the single player first.
