MK48 hands down was the easiest.
The HAMR was the hardest for me, but I didn't exactly have too hard a time with it. It's initial accuracy/clip size/bulkiness or heavyness or whatever made it worse than all others.
The third one, the LSAT I think it is, was the first one I used, and it's very much like the MK48, with faster shooting yet less accuracy.
The 2nd one, is basically an upgraded assault rifle. Has way better mobility, with a good reload time. Its speed and accuracy made for a pretty good time. It's tied with the LSAT for me.
In all honesty, I'd have to say the line up of the guns itself in the game is actually a fair description of how easy they were.
MK48 > 2nd One > LSAT > HAMR.
I will say that after getting nearly everything diamond as this point (I just need 1 more sniper and the shotguns), the first 3 LMGs are the first guns I've had all the blood thirsties done before the 150 kills with no perks/attachments. In fact, I think for the MK48, those 150 kills were actually the last challenges completed for the gold!