Generally speaking, Split Beam weapons are by far the most damaging. Their secondary fire does excellent damage, and can hit multiple opponents if used correctly. It depends a little on whether you need the ability to expose your own targets (in which case you'll need to use a weapon with an expose attack). My away team currently uses ALL split beam weapons, including two of my own, with a bunch of grenades and other random crap to trigger exposes.
Compared to split beam, nothing else even comes close. Probably this type of weapon will be severely nerfed soon.
but when it comes to guns I found a combination of wide beam pistols + split beam rifles tore apart mobs at lower levels.
At higher levels the exploit attack from split beam wasn't enough to kill, only to maim (still good mind)
Sniper rifles work best for single target kills even at high level.
So wide beam pistol (or any conical expose weapon) combined with either split beams or sniper will work best.
I would avoid cylindrical specials and targeted AOE as they tend to be a pain to hit optimally with. cylindrical you need the enemy in a single file line (more or less) and targetted AoE tend to do tiny damage and while good for the first shot (where groups are stood together) the moment they split up its lost.
That said I melee, it may not be "teh ubah" way but it works well and lets me get some entertaining fights out of the slightly broken ground combat.