MP7 w/ range & extended mags or ACR with red dot and impact or focus

Either the MP7 or ACR. Both are easy to use, and seem better than the rest of the guns.

The best gun is the one you are best at using.  Everyone is different.

Funny

On a more serious note:

Assault rifle:
ACR kick silencer: The best assault rifle for actually moving.

Even better in hardcore.

SMG:
MP7: Best SMG for varied short and long range combat, large magazine.
PP90M1: Best SMG for short range combat.
P90: Largest magazine for an SMG, good for that reason alone.

Shotgun:
Striker

Sniper:

MSR: Best sniper rifle
RSASS: Best low damage spammable sniper rifle

Dragunov: Best low damage precision sniper rifle (Patched <3)

MP-7 or the ACR,having said that I am still naff (but slightly less so)

Can't get on with sniper rifles and shotguns.