Guns are mostly low damage in comparison to swords, so you have to really think about the situations you're trying to use them for. Most people get guns just to deal with turrets, in which case they get an elemental gun - either an alchemer or the polaris pulsar lines.

In that case, polaris line (gigawatt pulsar) has 3 slow bullets that become strongest at range, with fast fire rate. They're okay at fighting crowds, but knockback is a little screwy with this gun.

Alchemers have 2 fast bullets, decent strength (when you use them effectively) regardless of range, but you'll usually have to pause between shots in order to avoid reloading, so I'd call it "medium" trigger speed. With alchemers, you are usually with status alchemers (weaker, but have chance of dealing status effects - Firo is nice when you're running often, Cryo can help you if you're soloing, Volt is good if monsters are grouped together) or the stronger non-status alchemers (Prisma for elemental damage, Shadowtech for shadow damage). Alchemer bullets ricochet to the left, so they're best when taking out crowds.

The Antigua lines have fast bullets, fastest fire rate, but weaker damage per bullet. This means you're usually walking slowly firing many bullets, so it eats your mobility. They are better vs few enemies than alchemers though.

Now, other than those guns, you have the balanced Blaster line, which has fast bullets, fast fire speed, but only decent damage (it's normal, so it doesn't have any effective damage situations). It tends to be better vs few monsters at a time though, instead of large crowds.

So in short:
Pulsar line: slow bullets, fast fire rate, very strong bullets (at range), decent vs crowds
Alchemer line: fast bullets, moderate fire rate, strong bullets, best vs crowds
Antigua line: fast bullets, fast fire rate, weak bullets, best vs few monsters
Blaster line: fast bullets, fast fire rate, moderate bullet damage, decent vs few monsters.

I omitted the Magnus and Autogun lines because they have 'slow fire rates'. Autogun>Needle shot has one of the strongest charge attacks in-game though.

NOOOOOOOO

No polaris recommendation without a good reason please. "Extremely spammable and kills everything fast" is not a good reason.
I dont want the forums to breed more polaris spammers.

Polaris:
Pros:
-Explosion pushes enemies away, making it a great side-arm for solo gunners.
-Explosion can prevent gun puppies from firing.
-With good practice, its possible to push enemies that are about to hit teammates. Careful precision is required. (IN OTHER WORDS NOT SPAMMING IT)

Cons:
-Most people tend to treat it as a spam weapon, which results in wild mob pushing. Considered the most irritating thing in game.
-The bullets are the second slowest in the game.
-Can accidentally trap teammates by pushing enemies into their only escape path.

Overall, its a good sidearm, but is used ineffectively by the majority. Most useful for soloing.

Strangely enough, I've yet to have someone complain to me about using polaris. Though the only time I've pushed an attacking enemy into someone else is due to an errant shield bump in the wrong direction. (and a bit of panicking.)

Note: Slightly less useful for trolling then the grand warhammer.

Blitz. Because it destroys absolutely freaking everything with a bit of dmg and ctr.