Whats your favourite MW2 Gun out of the WHOLE game. (Guns like the dragunov and W12000 DO count) So this would be the gun you would normally pick for online.  
 
Mine is probably the ACR or M4A1. 

Modern Warfare 2? Uh... 1887.

I had some favorites that I didn't use very much of online. Loved the Vector, the M4A1, and the FAMAS, but I stuck pretty close to the TAR and ACR online. 

MP5k extended mags

The SCAR. No CoD will ever beat MW2's cast of guns.

The Scar and the M4A1

I mean...wait...that game has more than one gun in it? I thought it was just different skins on the same assault rifle. Oh yeah, and a Sniper Rifle. And that one with the heartbeat detector. I like Sniper Rifles.

In campaign...Nothing. Pretty much everything was horrible.
In multiplayer, SCAR hands down. Intervention is a distant second.