I found that the best guns are found in the single player campaign. It's ashame they did not carry over some of these awesome guns. The programed code is already there it would be a cut and paste job for the most part.
Can't say 'best'. My FAVORITE is the HAMR.
Primary Gunfighter with attachments:
Quickdraw (necessary as the ADS speed is horrible)
Silencer (keep you off the map)
Laser Sight (for close quarters combat)
The HAMR has a large ammo capacity (75 rounds), very little recoil, great iron sights, and a solid fire rate.
Quickdraw is an absolute necessity and the HAMR is below average until you get it, but once you do it's an amazing gun.
MP7 with laser and reflex. When you play in hardcore even MP7 can turn to an Assault Rifle. That's why you'll need a reflex. And laser tightens the crosshair,so you can kill people by firing from the hip. Plus that,MP7 is a powerful weapon along with a big clipsize.
I use this gun all the time.