Well, lets see. CoD is great for story, but i find the multiplayer gets old after a month or so. BF:BC2 is great for multiplayer, but I prefer Halo 3. Yes, it is old. Yes, there are a lot of dicks playing, but the mute button exists on purpose. If you are creative, even the slightest, then Halo is the best choice because of its excellent map editor (Forge) and saved films (for Machinima). As for it's playlists, you can find just about everything, from hard core competitive (Ranked/MLG) to casual fun (Action Sack/Social). Also, if you hate that, but have friends, custom games is your best friend. You can edit everything about any game type and map to suit whatever you want to do for fun. Hell, you can run an Halo RP if you want.
To top it off, if you get ODST instead, you have all the DLC maps (Heroic/Legendary/Mythic I/Mythic II map packs), Fire Fight, Halo 3 multiplayer (Custom games and Forge included), an excellent (But short) campaign with a good story, and access to the Halo:Reach Beta in May. Pretty good deal for $60 bucks in my opinion.