I've been using the M4 since I got the game, and I've found the following to work for me:
M4
-Heartbeat
-Red Dot
(slowly breaking addiction to both of those. N00b crutches)
AA12
-Silencer (I only use shotty when I'm out of ammo with the M4, and at that point I'm usually hiding to unleash a killstreak)
-Red Dot
Semtex (get a lot of kills) and Flash
Bling + Stopping Power (M4 = weak gun) + Steady Aim IIRC
That setup has got me through a lot of levels I'm not familiar with. I'm slowly breaking away from it, but basically I just find people on heartbeat, throw a flash, shoot them, rinse/wash and repeat. Works great for long-range stuff too, I've found. I had a big time problem with aiming early on, and the M4 is a nice low-recoil gun.
Lately I've been using SCAR (Grenade and Red Dot) with the AA12 and a claymore. Another nice setup, especially if I find a building I like. I'm only level 33 or something like that, waiting for either the ACR or M16 - I don't like the 3 shot burst, but I sometimes pick up the M16 when I'm running out of ammo and just went on a 17-0 streak last night after grabbing someone's - I'm usually a 1:1 K/D ratio player, so 17-0 is pretty "wtf" for me.
And Akimbo P90s is just fun. Useful for courses like Rust or Highrise, where a lot of combat is close-quarters. Downside is that my long-range weapon is a freaking pistol. Not a setup I need constantly, but it can be fun.
Killstreak is always UAV + Predator + Harrier at this point. If I can get the UAV, I can start hunting guys and that usually works out fairly well.