MTAR: 40 max damage, average ROF. Easy to use but specialized at nothing.
Type 25: 33 max damage, high ROF, short damage dropoff makes it better close. Good for aggressive players because it kills fast when you're close.
Swat-556: 3 round burst. 40 max damage out to 50 meters (MTAR only 10 meters, for comparison) means 3-hit kills to 50 meters if all shots hit. Slow ROF, low recoil, noticeable burst delay. Slight lag means 2 bursts might be needed even if you swear all 3 bullets hit them and it's easy to get killed between bursts.
FAL: 55 max damage, 2-hit kills to 16 meters, low recoil. Pretty good with select fire.
M27: 33 max damage, average ROF. High accuracy and low recoil. Longer damage dropoff than the type 25 makes it better at range but it kills slower than other ARs. Silencer reduces kill speed even further, but this can be made up for with accuracy.
SCAR-H: 45 max damage to 43 meters. Low ROF (625) but strong recoil. Easy to control anyway. Long damage dropoff makes it effective at medium range even when silenced.
SMR: 50 max damage, but 2 hit kill range reaches to 38 meters, Higher recoil and slower ROF than the FAL. Don't bother with select fire.
M8A1: 4 round burst. 35 max damage to 6 meters, 33 damage to 30 meters means 3-hit kills to 6 meters, 4-hit kills to 30 meters. Once again, lag can make more bursts necessary. Much faster ROF and less burst delay compared to the Swat. Increased recoil when using select fire. Might be better using the M27 if you want a full auto version of this depending on your recoil control. Easy to burn through ammo.
AN-94: 40 max damage out to 30 meters, slow ROF and low recoil. Special effect of the first two shots firing at 900 rpm and then the following shots are 625 rpm. Much longer damage drop off compared to MTAR. Lower damage and shorter dropoff compared to the SCAR-H. Great with select fire because repeat bursts are actually faster than full auto.
I'm trying to get all of them leveled up. My favorites have been AN-94 + Select Fire, SMR, and M27 + suppressor + foregrip. I tend to play carefully and get people from further away when using ARs. I've also done well with the M8, but I feel like that's just because I'm shooting a lot, not because I'm shooting well.
M27 is the easiest all-around weapon to use - medium range, medium damage, low recoil (possibly low enough i would say it needs more)
FAL/SMR are great for longer range, especially with select fire - they turn into laser beams essentially.
Type 25 is another great all-around AR - high rate of fire with its damage per bullet mathematically produces the highest damage per second output in the AR section. this high RoF also means it performs better at close range than most of the ARs.
AN-94 is the closest thing to the previous AK47 variants. once you learn to master it's unique first two bullet damage, you can burst people down quite quickly. recoil is also quite low and easy to manage.
MTAR is also a decent all around AR but i'd say it's outclassed by the Type 25 given the similarity in stats, but it somewhat comes down to personal preference on that one.
M8A1 has very high RoF for a burst rifle with very low recoil and absolutely no delay between bursts. with full auto, it's even better up close than the Type 25.
the SWAT-556 is okay if you really like burst weapons, but i've found it to be fairly inconsistent and the iron sights not the best for use with the burst mode. full auto it is quite strong however.
SCAR-H is very high damage just like it was in MW2, with a bit of additional recoil compared to its predecessor. however, with a silencer and some controlled burst firing it is still quite a fast killing weapon despite it's incredibly slow RoF to compensate the damage per bullet (a fore-grip also significantly decreases the vertical recoil).
on the whole, the best AR i would recommend would have to be the Type 25 given it has very easy to use iron sights, so just about any attachment of your choice can be effective (especially a silencer given the RoF). the high RoF makes it much better than the other ARs in CQC putting it almost on par with many of the SMGs at point blank range. additionally, it has very low recoil despite this RoF and so can also be used effectively down longer ranges, even at full auto if you have good enough aim. i also found this gun to be particularly easy to pick up compared to most of the other rifles and it is available right away, which is another plus.