My tips:
AR is the best All around weapon. it is solid in a catagories and can hold it's own, but in every catagory there is one gun that's better than it.
DMR is the best long range for health damage. Carbine for Shield damage, Light rifle for balanced.
Same for the automatics: AR health, Storm rifle Shields, Suppresor Even.
Frags are good for area damage and suppression, Plasma for kills, Pulse to cut off routes.
Find a loadout that complements your play style, then tweak it to fit the various map types.