I used to think it was the LSAT then moved to Hamr and thought it was the Hamr, now I tried the MK48 so right now I think its that..
This basically happened to me, when I was cycling through them all for diamond I kept finding a new best, but now I almost completely run LSAT w/ stock, QuickDraw, foregrip then lightweight toughness and tac mask.
For typical LMG long range supression and area denial, Mk 48 has no equal. FMJ is the one mandatory attachment at all times, cover will mean nothing. Then use and combination of Dual Band (my choice), Target Finder, Suppressor and Rapidfire. You will still kill in 3 hits over every sight line in the game, but RF makes you fare more deadly in mid and close distances. Plus, you burst fire at long range anyway for the 2hit head shot.
For all purpose and more standard death match gameplay, LSAT is most well rounded. Great mag size, great irons, good fire rate, good range and good penetration. If you want to do everything well, pick this. Stock and Quickdraw are all you need, but suppressor is a good choice too, as is FMJ.
And for the bulldog bullet hose its the QBB LSW. This one is great for hardpoint and headquarters. Laser Sight gives you great hipfire, and suppressor eliminates muzzle flash. Stock and Quickdraw help also.
You should run Toughness on everything. Mk 48 needs hardline because youll pick off so many enemies from afar youll rack up high streaks easily. This makes the FHJ a great companion for extra points, and use shock charges to protect your rear and alert you to enemies. The other two benefit from Flack Jacket and Lightweight combined.
i can't tell you anything about the best LMG setup, cause then everyone will just emulate it. find you're own best setup. try using the LSAT or HAMR first, those are good introduction LMGs. the first two require some experience to be used effectively.