It depends on your play style. I have logged quite a few hours on all of these guns.
The SMR has a tiny magazine at 20 rounds and the slowest rate of fire of all the assult rifles since it's semi auto. However, it 2 hit kills all the way out to the far end of mid range. It 3 hit kills all the way out.
The m8a1 is a 4 round burst assult rifle with a 32 round clip. It fires incredibly fast and has excellent accuracy with very little recoil. However, it has a lower damage threshhold so be prepared to use 2 burst to get a kill. It's an excellent weapon at mid to far range
The An94 is probably the most versitile assult rifle in the game. However, it has some drawbacks and a steep learning curve. The frist two rounds out of the weapon every time you pull the trigger come out at 900+ rpm. However, after the frist 2 rounds it drops off to around 650 or so. So, the trick with the gun is to fire two round bursts by quickly tapping the trigger and letting go. Also, the gun has some strong recoil so controling it's rate of fire is critical.
Really, this choice is up to you. It all depends on your play style. If you are a precision shooter who preferes shotguns or sniper rifles, then the smr will feel right at home in your hands. If you are a tactical player with some experience, then grab the an94. If you are newer or just want something that is easy to learn and use the m8a1 is your best bet. Good luck. Whatever you don't pick this time you can always choose next time