-The AR's (Assault Rifles) are an even mix of sniper rifles and SMGs. While only sub-par in close combat, their single shot is effective at a distance, but their power lowers their accuracy when set to automatic. Among the more dangerous AR's are the FAMAS, G36C, M8 and MTar.
-Between the LMGs, shotguns & snipers, it all becomes a matter of preference. Note, though, that the LMGs are highly inaccurate and, in my opinion, are a waste of time.
-Also, overall, it's wise to give each gun a try and find one you like, then keep using it. The more you use a gun, the better you become with it. Learn your weapon.