What is the best weapon on cod6

It really just depends on how you want to play:
If you want to rush, you can use silenced SMG's and shotguns or pistols.
If you want to be a loud, explosive freak, using LMGs and Assault rifles with Rocket launchers like RPG's is good.
If you are a stealthy sniper, use the WA2000 or the M21 EBR or B.50 Cal.
If youre a talented sniper, using the intervention FMJ or extended mags.

The strongest Assault rifle:
AK-47 with fmj.
SMG:
UMP45 with silencer
LMG:
RPD with grip
Sniper:
Barret 50.cal with fmj
RL:
Rpg