XM8 Compact - BFBC2. Love it. The iron sights, the rhythm, the fact you can full-auto hipfire it close range and it's still lethal (unlike most BC2 guns which genrally spray everywhere but the target when fired full-auto).
40mm shotgun attachment - BFBC2. Not strictly a gun I suppose, but it's a great piece of kit. On maps like Oasis this is almost my primary half the time. Couple it with a high accuracy rifle like the AN-94 or M16A2 and a 4x scope and you can pretty much rape anything on the battlefield that walks.
870 combat/w 12G slugs - BFBC2. slugs basically turn shotguns into sniper-shotguns in BC2. Out-sniping 12x scope toting wookies with your unscoped shotgun is a pure joy. Oh, and the c4 you get as an assault for taking a shotgun is an added boon.
EMP shotgun - Transformers WFC. Loved the MP in this game, totally overlooked gem. The EMP shotgun (basically a double barrel shotgun Cybertron'd up)was the finisher in my favourite combo for the Scientist (i.e. Jet) class. Swoop into a single or group of enemies, transform, melee, shockwave ability, drain ability, fire shotgun at anything still moving, transform, fly away. Devastating.
Bolt Action Rifle - RDR. In RDR I was a non-deadeye headshot guy. pop up, headshot, back down, pop-up, headshot etc etc. The power and slow speed of the bolt action works quite nicely with that approach.
Engergy/Alien weapons - FO3. They turn guys into piles of goo/dust. That's win to me.
Nova (silenced) - Crysis 2. I only recently realised you can fire a silenced Nova while cloaked and stay cloaked. Totally changed the game for me. I sneak around popping heads with my silenced Nova. stick a laser sight on that bably and with the suits aim stabiliser you can get a headshot from a good distance away.
As for the SCAR, I hate the SCAR-L (SMG variant) in BC2. It's sort of in between an assault rifle and an SMG, but not really as good as either imo. I remember it (SCAR-H assault rifle varient) being ok in MW2.
I'd also like to know if StolenGlory uses the Irons on his BC2 M60 or if he sticks optics on it. Can't stand the M60 sights myself. It's making the 100 kills in Nam a real grind because I just feel like I can't hit anything with the damn thing unless I hipfire dudes in the back.