Remember this is my opinion and leave your opinion down in the comments.
Let's start with the assault rifles. Best: AN-94. It can kill a person with three bullets if all of them hit a player. It is also cool looking. Two other rifles that were close were the Scar-H and the M27. Worst: SMR. The worst assault rifle in multiplayer in Black ops 2. The sound of it is awful and it's just a piece of shit. Other rifle close to this position were the FAL OSW. Now here are the shotguns
Best: KSG. Just a beast for fast and accurate kills. One accurate shot and the enemy is dead. The other contender was the Remington. Worst: S12. To be honest none of these guns are bad, but I had to pick one and it was the S12. It has the boring look and I don't know. Here are the LMGs
Best: LSAT. The best LMG in the entire game and maybe the entire series. It is just so accurate and perfect. Worst: HAMR. Well, you guessed it. It has so much recoil and I know some people have had some success with it. It is rare to see online now and it just sucks. Here are the Sniper Rifles
Best: DSR 50. The best and most damaging sniper in multiplayer.
Worst: XPR-50. I never liked this sniper rifle and I have never really used it either cause it sucked so much. There's nothing else to say
Here are the Sub Machine Guns
Best: Skorpion-Evo. It kills so fast with such a fast fire rate. It's just so powerful. One other powerful contender was the MSMC
Worst: Chicom CQB. The one and only three round burst gun in Black ops 2 Multiplayer. Well, what did you expect on this list other than this gun. I don't like a lot of three round burst weapon and this is one of them.
The Pistols Best: B23R. Now last time I said I hated MOST three round burst weapons. But, this gun is amazing as a pistol to be three round burst.
Worst: Executioner. I tries to be a shotgun and a pistol at the same time. But, sadly that does not workout. It just needs to go back to the to western days.
The Launchers Best: RPG. I had to choose. I never used this gun since it was never really needed for anything.
Worst: FHJ-18 AA: With the RPG and the SMAW I could at least kill some enemies and take down planes. But, this is used ONLY for taking planes down and nothing else.
Best of the Best: AN-94. Will remain as one of the best guns in the series.
Worst of the worst: FHJ-18 AA. Sorry I had no other choice. The worst guns have at least a chance in killing an enemy, but this will never have a chance to.