Assault Rifle: Type 25 (the crazy fast fire rate means it matters less if you miss shots)
SMG: Chicom CQB (the second fastest killer in the smg category combined with extremely low recoil)
Sniper: Ballista (it has lower damage than the DSR-50 but the ads time gives me the advantage over other quickscopers)
LMG: QBB LSW (this has an extremely fast fire rate with a 3 shot kill potential)
Shotguns: KSG (I know this can be unreliable at times, but a skilled player will kill with it)
Pistols: B23R (no other pistol can put 3 bullets in an enemy as fast as this one)
Launchers: SMAW (this can tackle ground targets as well as enemy drones and scavenger makes this really effective)
And the attachment that I use all the time is quickdraw. Fast killing time won't matter if the the enemy can start putting bullets in you faster than you can.