They are pretty well balanced, to be honest.
In general, the SMGs are fairly powerful, the ARs are not nearly as dominant as you'd think, and any grenade launcher is a support weapon only. You can't go killing groups of enemies with them.
In particular, I've been enjoying the Bulpdaun SMG and the Euston AR. The Bulpdaun has a high rate of fire, excellent stability (better than any other SMG hip firing or ADS, I think), but has somewhat bad damage (don't expect more than one kill per clip). The Euston has slightly worse stability, but the damage it puts out is mind-bending (I've been playing almost exclusively medium, I haven't even tried light yet, lol).
Oh, and shotguns are actually really good as a primary weapon. Definitely give those a try.