I mainly use the M4A1 which is the default weapon under US engineer. I mostly prefer that because it has faster ROF/ less recoil (Helps me burst 4-5 bullets at medium range too) although the bullet damage is lower than other weapons..
If you check my stats
Battlelog / Sign in here, you can see that I have service stars for M4A1/A91/Scar H/Aks 74(almost)..Am a gun whore love to test every weapon..
I would suggest you to master Scar H(if you can live with 21 bullets in your magazine), its currently the best weapon in the game regardless of any class, because of the insane amt of bullet damage..Although this will be nerfed in the next patch..
As for A91 its jus ok..I dont quite like its unusual style of recoiling..And it sucks balls with suppressors(bullet velocity becomes too slow).
P.s. AKS 74 is actually good too..Damn I jus love every weapon!