I've been loving the M8 since I first unlocked it and subsequently chose it as my first unlock. I used the M27 & T25 when the game was initially released but haven't used either much since. I always prestige as soon as I can so I haven't spent much time with the SMR or the AN94. I'm currently working on the FAL challenges after picking one up and destroying with it.
Tier 1 - Fal - SMR
Tier 2 - M27 - M8A1 - AN94
Tier 3 - SWAT - SCAR - MTAR
Tier 4 - Type 25
IMO only the Type 25 really doesnt have any major advantage. Its meant to be played like a an AR/SMG hybrid...but it generally looses to SMGS at short range I've found, and its recoil/damage ratio can make it a tad weaker at longer ranges.
Some might wonder why M27 is above the Type25 at long range given the 25's fast firing rate...but it comes down to one thing. Accuracy. m27 is an automatic laser capable of sniper precision auto. You can literally square off against snipers at range and win if you aim right. If you hit before they fire, you will flinch them and they will likely die unless they duck behind cover quickly. M27 super accuracy also make it a flinch-inducing machine. Even if they have toughness you can keep their bullets off you if you hit them first.
Tier 3 can all be great, but have some major drawbacks. Mtar can be a little wonky with its bullets at longer ranges. Swat needs a perfect upper torso hit, and its recoil is insane at full auto, and the scar is strong, but has massive recoil as well, making it require a some extra focus to use.
Tier 2 are all good.
Tier 1 are So good they might even be slightly overpowered. A well trained Fal/SMR user almost always has the advantage. They also kill insanely fast.