May I please have YOUR input?

Im trying to find a new multiplayer game to get into. I played COD4 for awhile but got bored of it and traded it in. I'm just really clueless in what I should get next. I'm mainly into FPS games. I've played ( Gears, Shadowrun, and COD4 ) just to name a few. I'd like to have something with a decent single player but I'm more focused on something online. Any suggestions?

(NOTE : I'm not into the Rainbow Six Vegas 1 or 2. I've played them but didn't really enjoy the multiplayer aspect at all. They just seem too slow and tactical for my taste.)

Thanks

Forgot I played Halo 3 also.