TAR RDS Or Silencer
Computer Specs: Processor : AMD Phenom II X4 975 @ 4.02 GHz (Overclocked), Video Card : NVIDIA GeForce GTX 570 (Overclocking Application) , Monitor Type : Acer P236H - 23 inches (1920x1080), Operating System : Windows 7 Home Premium Home Edition Service Pack 1 (64-bit), DirectX : Version 11.00, Windows Performance Index : 5.9 on 7.9 lol.
My honest belief is that no matter what, it's your skills that matter. I mean, a professional player with an F2000 and Scrambler can kick a noob's SCAR-H and Stopping Power any day. With the exception of the F2000, I think it's all about the iron sights and how personally comfortable you are with them. Sure, I guess there are some differences and advantages on one gun over the other, but not enough to greatly improve your gameplay.
Anyway, that being said, here are my favorite 3.
M4A1 (FMJ) - Simply because the Iron Sights on it is extremely accurate for me. All the other attachments for the Assault Rifles seem pretty useless for me. I refuse to use the Grenade Launcher unless I get really pissed off. I don't know--I get the habit of trying to get 'legit kills.' Since the Iron Sights on the M4A1 are accurate already, it pretty much cuts off the Red Dot Sight, Thermal, Holographic, and ACOG....You should never really use ACOG on medium to high recoil guns anyway. Masterkey is a piece of crap except in Hardcore games due to its insane range...so FMJ it is. Silencer is good, but not great...I mean, even though the M4A1 has low recoil and high RPM, its damage is low enough already.
Intervention (FMJ)- The Barrett, WA2000, and M21 have the same scope zoom range whereas the Intervention has an increased scope range. Don't believe me? Try them out yourself...you shouldn't be comparing the Barrett to the Intervention like a lot of people do--they are two totally different sniper rifles. The M21 is a bit better than the WA2000, looking at sight time, drop/raise time, and recoil. Thermal is okay, only on Wasteland. Extended Mags is useless because I go Sleight of Hand anyway, and it's a Bolt Action for Chrissake. I'm not a big fan of ACOG on sniper rifles, though I know people who use it and they do extremely well, despite criticism. Silencers, only for Hardcore...damage reduction. Heartbeat Sensors, I don't like using. Personal preference. I get UAV's anyway, and don't underestimate the UAV...and the Counter-UAV. It's UAV, Counter-UAV, and AC-130/Chopper Gunner for me. :P
UMP45 (Red Dot Sight) - I hate the Iron Sights, so the Red Dot Sight goes with this one. Despite it's lack of range due to its recoil, it's actually the most powerful SMG in the game...mathematically. The more damage a weapon has, the more you'll get out of Stopping Power. I use it with Sleight of Hand Pro and Lightweight Pro anyway, since the UMP45 is strong enough, I use it for versatile reasons. It has medium RPM, so if you miss, you lose that 0.1 second advantage of killing your opponent, assuming he shot you at the same time and didn't miss. The higher the RPM, the more you can make up for your inaccuracy, since weapons with high RPM usually have low recoil. I don't really use Rapid Fire because it causes extra recoil, which is unnecessary...only use it with the Vector, close-range. Why? Low-recoil, vector has the highest RPM, so boost it with Rapid Fire is godly, fastest RPM in the game, and Stopping Power. All power stuff, but low accuracy because of the damn Iron Sights, but Bling with Red Dot might make up for it because it's an SMG and you don't really need Sleight of Hand due to SMG's fast Drop/Raise, reload, sight times. Silencer on the UMP45 is very good. Whoever uses Thermal on it is a douchebag...TOO MUCH RECOIL. For a weapon made for close-range, why turn it into a long-range?
If you were to ask what was my fourth favorite, it's the FAMAS.