The best overall guns would probably be MSMC or PDW because of their ease of use. FAL semi auto is GREAT if you have trigger finger. Scar is also a really good gun.
I personally like MP7 the best by far. Fast fire rate, recoil that is easier to manage than you'd believe, high damage, good range. Also the 40 clip size and quick reload means you don't necessarily need fast mag or extended clip. Iron Sights are probably clearest of all SMGs so no need for red dot. So Stock or Suppressor is great attachments. So for me MP7.
I find Hardline to be a good perk since I use high streaks. Ghost is good for going into spawns and Flak Jacket is great for objective modes. Dexterity is a good perk but honestly if I have to choose other 3rd perks I would say Tac Mask is a must. The amount of shock charges and tac nades slowing down my rushing is far more detrimental than firing slower after sprinting and climbing in windows.
Originally posted by COOKIE
PEACEKEEPER
Attachments: Stock, Long Barrel, RedDot
Perk 1: Lightweight, Ghost
Perk 2: Scavenger, Toughness
Perk 3: Dexterity
(You can replace Ghost with Flak Jacket )
Lightweight isn't nearly as useful as Flak Jacket, Hardline or Ghost. Also peacekeeper is at best 3rd best SMG and that's only if you add rapid fire to it.