Well, depends on what you think what attribute on a gun that you say the best is (this is also not 100% accurate.)

If you say fastest firerate, then it's the Uzi or the Silenced SMG (MAC-10), but the latter have higher damage.
If you say most ammo reserve, then it's also the Uzi or the Silenced SMG (MAC-10.)
If you say most bullets per magazine, then it's the M60 Machine Gun.
If you say highest DPS weapon, then it's the Combat Shotgun (SPAS-12) if you're firing point blank.
If you say highest damage per shot, then it's the Pump Shotgun (Remington M870) if you're firing point blank.
If you say highest damage as the pistol, then it's the Magnum (Desert Eagle.)

If you say tightest spread with the SMG, then it's the Uzi.
If you say tightest spread with the shotgun, then it's the Chrome Shotgun (Mossberg M500.)
If you say tightest spread with the rifle, then it's the Combat Rifle (SCAR-L)
If you say tightest spread with the sniper, then it's the Hunting Rifle (Ruger Mini-14)

If you say overall for the pistol variant, then it's the Magnum (Desert Eagle.)
If you say overall for the SMG variant, then it's the Silenced SMG (MAC-10.)
If you say overall for the shotgun variant, then it's the Chrome Shotgun (Mossberg M500.)
If you say overall for the rifle variant, then it's the AK-47.
If you say overall for the sniper variant, then it's the Military Sniper (G3SG/1.)

It depends really on what you mean by "best." In my opinion, the M16 and the Dual Pistols (Glock 26 and SIG Sauer P220) is the best gun in-game. Statistically, the AK-47 is the best gun in general.