Plasma Pistol/Rifle work wonders in the single player campaign, the charge shot from the pistol drains all shields from any unit, fantastic for lengendary setting. Rifle works ok in multi, I've found it causes the one who is being hit to move/turn much slower great irritation weapon.
Pistol I always use, great medium-long range gun. I allways play the map tombstone, good medium range map. Shotgun works wonders with multi player.
Never been much of a rocket fan, good for laughs and anti tank guns, but that's it for me, too slow and easy to dodge. Sniper is ok...but easy to get into sniper fights and I've found sniper guns easier to dodge nowadays after playing for over a year.