There isn't one. Depending on an individual's play style there may be a few which are best for that person, but there's not one "better than all the rest all the time" weapon in this game. It's great.
EDIT: From what little you said about your play, PDW w/suppressor and either laser sight or quickdraw would probably be great for you. That was one of my early favorite combos.
Harv72 is right there is no dominant weapon in this game. There are a select few that really stand out from each class. Since you play a sneaky play style I will assume you like to run ghost and silencer.
Since ghost now requires you to be moving it goes great with SMG and shotguns, although AR still not a bad choice if you want some longer range capability.
SMG: I would say PDW until you unlock MSMC both have pros and cons. Vector is great in hardcore.
Each weapon has a range advantage and disadvantage, but the maps make the SMG dominant and the accuracy of the MSMC make it the best weapon overall.
Best is a matter of opinion, and perhaps a camper might say AN-94 or M8A1, but the MSMC is generally the most consistant weapon you can use if you are moving around.
Its all in preference on what you like. The best guns by rate of fire, damage, and bullets to kil (knock down force) is the MSC and the POW. Snipers is XPR 50 and DSR by damage but others is prefrece. Damage in aswsult rifles is SMR by far is the best along with M27. My favs are M27 SMR MSC XPR Balissta and MTAr but thats prefernce and its all different on what YOU like. You might be a rusher or a camper. Different guns are needed for both. Hopefully this will help
Captn Valor Prestige 2, BAMF
As true as personal preference is, sometimes certain attatchment combos make guns truly OP, maybe not the best gun in the game, but certainly a force to be reconned with.
Here's some of note:
DSR 50 with FMJ makes it so there's less skill involved with a one shot kill. Just don't hit them in a limb.
SVU with FMJ and ACOG is OP if your trigger finger is fast and you can compensate for recoil.
Chicom CQB with Rapid Fire, Grip, and Target finder feels like a contender for OP. Haven't unlocked it yet so I can't test it.
Scoprion with Rapid Fire, Grip, and a sight is scary up close O_O
SMR with all damage boosting attatchments might be OP but then again haven't unlocked it so I can't test it.
Any LMG with Target Finder and FMJ... there's a reason it's a default class.
KAP 40 dual wield.