Alright, I'll give you the basics.
Piercing
Magnus Line - Fires brutal shots capable of stun and interruption. The charge attack does high damage(7x damage) against enemies that are facing you.

Piercing Blaster - Lame gun. Its just easier to use.

Blitz Needle - This powerful weapon can output extreme damage, but you can't move while shooting.

Shadow
Tundrus Line - Shadow Magnus is fine and all, but it's freeze affinity debilitates the charge that the Magnus is known for. It's unlikely that you'll be able to get all 7 Charge Hits on enemies.

Permafroster Line - Pulsars are used for knockback, but the Permafroster's freeze makes it a little bit harder to knock back enemies. But who cares? It shoots mini bombs.

Shadowtech Alchemer - Eh. I'm bad with this gun.

Shadow Blaster - Also lame.

Catalyzer - Nobody cares, end of story.

Antigua - Obsidian Carbine may be capable of doing decent damage with its poison, but it still just shoots wimpy BB shots.

Elemental
Pulsars - Shock or Fire? Eh, these guns shoot mini bombs and have knockback. Expect it to be powerful against crowds.

Alchemer - Fires ricochet shots, which requires practice to master. Also only has two shots.

Elemental Blaster - Lame.

My opinion? I dont have one, because I'm a sword user. Pick something that fits your playstyle best.

If you're a gunner, you should start out with drivers. Make a shadowtech and one of the 4 elemental ones.

Normal dmg weapons have the benefit of never doing terrible (gray) dmg against monsters, but they'll never do as much dmg as a shad/ele/pierc wep would. It's still better to have 2 different spec dmg ones, like shadow+ele, that way you would do great dmg vs 4 monster types, and normal dmg vs 2.