Well, the Halo games (2, 3, and Reach are the ones I've played online) are among my favorite. Definitely my favorite FPS to play online. For co-op, it's definitely Resistance 2's online co-op. But I think the overall winner is... Warhawk. I played that game to death back in the day. The online is a ton of fun. And I'm still waiting for the sequel.
...And if it wasn't for Warhawk, VGC, and real life friends with a PS3, my friends list would be empty. Warhawk actually contributed the most to my friends list.