There actually have been favorite weapon threads before.
Personally, I can't really pick just 1 single weapon above all the others, so I'll go by class.
Assault Rifle: FAD or AK-12
Sub Machine Gun: Vector CRB
Light Machine Gun: M27 IAR
Shotgun: Bulldog or FP6
Sniper Rifle: Lynx
Marksman Rifle: IA-2 or M14 EBR
And I think it's safe to say that most people will have the AK-12 or Remington R5 as their most used weapons, lol.