I like the Silenced FAMAS w/ Extended Mag. It's just so all around useful. It helps you be sneaky in case you're trailing a victim or trying to snipe with it. It's very accurate, and the rate of fire assures that it's fatal when someones in your sights. In Hardcore TDM I love the Python w/ Speed Reloader because it's a very accurate one-hit kill sort of weapon.
I'm pretty good with the FAL in MW2. I remember the first match I used it in (having unlocked it the match before) I got like 25 kills with it and was the MVP of the match. I like it with the holographic sight solely for the quick pop into ADS and improved accuracy over red dot.