best weapon?

It all depends on your style of play, whether you like to rush in and get the kills or sit back and play safe.
If you want to rush i would use the the UMP45 with the silencer attachment. (Always use a silencer with this weapon, it makes it much more accurate and therefor easier to use) For your secondary weapon i would use the SPAS-12, with grip as your attachment. Your perks for this class should be Marathon, Stopping Power and Commando. Stopping Power is a must have. Marathon and Commando are just useful/handy perks and there pro's are really good.
However, if you prefere to sit back a bit then you should probably use either the M4A1 or the FAMAS,(As they are both easy to get long range kills with) both with the red dot sight attachment. (Personally i find that with both these weapons, if a silencer is added, the damage is noticably reduced) As for your second weapon, i would say the SPAS-12 again. It is very useful to have a shotgun, as it is handy when you go into close quarters with the enemies. (Buildings or their objectives/flags) As for perks, i would use Scavenger, Stopping Power and either Commando or Ninja.

the best guns are scar with fmj and spas with grip perks should be scavenger stopping power and ninja pros

personally i cant go past the G18 akimbo i chop and change my primary weapon on a daily basis depending on what i want from sniper to assault to SMG it all comes down to personal preference. also as far as perks go you need steady aim and scavenger as G18 akimbo chews bullets i also use stopping power for the extra damage. but at the end of the day im just telling you my personal preferences and you will have your own go through all the weapons and see which one seems best for you.

My signuture weapon is the M4A1 with fmj and heartbeat sensore with the m1041 shotgun with silencer. my perks are 1. the fast reload one. 2. cold blooded. 3. commando