Maximum PC

Win.

To answer the original question, I use the following weapons depending on the map:

Close range, storming buildings: FAD
Medium range: ACR6.8 or M14 ... depending on whether I feel like sniping
Long range: M14 or AS50 .. depending on how mobile I need to be.

Assault Rifles: M16A4, SCAR-L, CM901 and MK14.
Sub Machine Guns: MP5, UMP45 and Mp7.
Light Machine Guns: L86-L5W, MG36 and MK46.
Sniper Rifles: L118A, Dragunov, AS50 and RSASS.
Shotguns: USAS-12.

i prefer Knife as the best weapon in MW3 ..

I haven't got to play to much yet but at close range (1 - 5 meters) It's pretty much any dual wielded SMG with steady aim. I haven't had to much experience with all the assault rifles yet, the FAD is OP from what I understand. All I really care about is the fact that the M16 has been nerfed to hell even worse then MW2 it seems. Long range for me is the L118A but the MSR is supposed to be just as good. 12 levels till I get the M14. *Sigh* Why'd they have to put the only semi-auto single shot rifle at 60?