SMG: MSMC. yes it has less damage and ROF than the skorpion, but the skorpion's very limited range makes it an extreme close quarters SMG in which you might as well use a shotgun. the MSMC has a good balance of power, ROF, and range.
Assault Rifles: AN-94. in regards to rate of fire, it is rather slow. however, that also means you dont dump 3/4 of your mag on a single enemy with 60% of bullets getting lost in the lag cloud. it has moderate recoil with high damage and excellent iron sights.
Shotguns: R870 MCS. this is the boss hawg of the shotguns. combine with laser sight and long barrel and you have one hell of a killing machine. the KSG has a much longer OHK range but isnt as effective at close range because of the hip spread of the SINGLE slug it fires. the KSG is more of a novelty skill weapon.
Sniper Rifle: Ballista. yes the ballista is OHK from the chest up and the DSR is from the stomach up, but treyarch actually did a good job of localizing the hit boxes to where the chest actually extends down farther than nipple level. it isnt the hitmarker machine that other chest up snipers have been in the past. you also get 7 rounds instead of 5 rounds that the DSR has.
LMGs: who gives a shit? you run like a turtle with downs syndrome.
pistols: TAC-45. treyarch did a good job balancing the secondaries with the pistols being high in damage and the machine pistols being low in damage. the TAC-45 kills in 2 hits at very close range and when paired with laser sight or the fast mag attachment, it is quite effective.