I usually just use random weapons (specifically different rifles and SMGs), but my best seems to be the Scar + Heartbeat

scar or mp5k

"scar or mp5k "

Mine is the M16 with bling perk with ACOG scope and silencer to me it can be a good a medium to long distance weapon

UMP45 and ACR. 

I enjoy the AUG Bar and M16. I used to really like the scar, but after switching to the M16 it seems hella inaccurate.

AUG with the fore-grip.

FAMAS or UMP

Im 4th prestige and i just started using the FAL, in hardcore its pretty beast.

I switch around between all of the assault rifles with the exception of the FAL.

FAL with red dot.

M16 with Red dot  and Akimbo Rangers as secondary.

FAL, ACR and AUG are pro.

m16 with heart beat

ACR, UMP and the intervention are the most common. Occasionaly I rush with akimbo mp5ks though, that's usually really fun.

Right now I use the m16 with red dot. Before that, I really liked the SCAR. But every time I see an ACR (especially with red dot) on the ground, I pick it up. I love that gun. I don't play a ton, so I'm only about lvl 44, so soon I'll have it all the time.

I seem to have a number of "favorite" weapons, but it depends on the other team's play style and what map we're playing.
 
Basically, in my rotation (and I probably use them all equally)...
 
SCAR with fmj (especially with a silencer, or for longer range shots until ACR is unlocked)
TAR with fmj (flat out drops dudes, and better at closer ranges)
ACR with holo (LASER BEAMS!)
UMP with holo or rds (like using for better run speed, and clip is large enough to kill a number of people without using scavenger)
RPD grip and fmj (for walling and sniping)

FAL + Fall Colour + ACOG Scope + FMJ = The Sexiest gun combination.

The M21 EBR with a silencer has been working out pretty well for me.

M16 with Acog or Holo, and silencer is the best

Vector for FFA and TAR-21 for everything else. 
 
Also RPD+Foregrip /AT4 +Scavaneger Pro|Stopping Power Pro is a great class for Domination.

SCAR with silencer, I find you actually don't really need the red dot or acog sights.

FAL with FMJ and akimbo G18's.

WA2000 with thermal. Only gun I have fall camo on. Otherwise I like M16 with red dot. UMP 45 with silencer is cool too.

Scar-H w/ Extended Mags or FMJ 
ACR (Masada, it should be called) w/ RDS 
Spas-12 w/ Grip 
M93 Raffica Silenced 
Intervention, Barrett .50 Cal w/ FMJ

AK-47 
But i thought everyone knew the best gun in the game was the ACR. It really is like the Skorpion, because it has almost no recoil. 

Normally Tar 21 with Holographic, but at the moment I'm sporting an Ak-47 with grenade launcher.

Love the ACR with holographic sight

Tar 21 with silencer. It just feels so right for some reason

FAL, P90, RPD, and Intervention are my definite reliables when it comes to their respective classes.

M9 Akimbo with silencers. 

Intervention, MP5K and Akimbo Desert Eagles.

M21.

I'm usually with a shield and m9 tac knife, if I haaaaad to use a gun, then I'll go with the ak47.  I still hate MW2 though....

Sniper : Barrett /w Thermal
Assault Rifle : Tar-21 /w FMJ
LMG: L86 LSW Heartbeat
SMG: P90 Rapid Fire
Shotgun: M1014 Extended Mags
Machine Pistol: G18 Akimbo
Handgun: Revolver Akimbo
 
Overall:
Primary = Barrett /w Thermal
Secondary = G18 /w Akimbo

SCAR with silencer and FMJ (to be replaced with extended mags when I unlock it), with a .44 magnum (softcore) or an M9 (hardcore) 
 
MP5 with silencer and same sidearms as above 
 
M14 EBR with silencer (hardcore) or heartbeat sensor (softcore) and FMJ, with a raffica (hardcore) or a SPAS (softcore) 
 
for an anti-air class I usually go AUG or RPD with a Stinger.

" ump45 acog "

UMP45 for sure!

acr

P90 Silenced or assault rifle silenced with ACOG

" FAL with red dot. "