I DON'T SEE TEAM FORTRESS 2 ON THIS LIST! Well, essentially, here are my opinion on these games.
Halo 3 I enjoy playing with my friends who have yet to realize the wonders of PC gaming. I have fun playing it and I enjoy whacking the duckshit out of people with the gravity hammer. I've developed a bit of an aversion to it, mainly because I can't stand the fact that people will say "Spartan Laser" or "Gravity Hammer" but not "Battle Rifle". I hate the acronym BR with a passion, though I'm not exactly sure why.
Gear of War is fun when I'm pissed off and enjoy the idea of running around with a testosterone-powered chainsaw and a soul patch. It is without any doubt a fun game, but still feels kind of generic like Halo.
Call of Duty I have little opinion on, as I haven't played it much, but in general it does seem to be well-made and fun to play. Too bad I have to play it with the XBL community.
Indeed, serious multiplayer gaming arises on my part only for TF2. No game is more fun online.