What are the best weapons to use and upgrade?

I have not finished the game yet but my favorites are:

1) the minigun, which has increditable fire rate and decent damage and range,

2) the taser, which has great damage, range and the electric current bounces between enemies,

3) the plasma gun, which has great range and also bounces between enemies. Ultimately, the best weapons are more rare and you don't get them immediately. Unfortunately I haven't unlocked any of these as a default weapon yet so I use an upgraded shotgun. Honestly the game is well balanced so you can do well with virtually any weapon combos.

Shotguns are good for close range carnage but terrible at a distance. Sniper rifles are ideal to kill from across the room. The automatics are a good choice for a quick hitting shot, although it is weak as tour only weapon. The throwing knives do good damage at close range but take their time returning to you. You can even use melee weapons, which offer good damage for a low price at a cost of range. Try to earn/buy a full compartment of weapons asap, to maximize your power. You can always merge/sell them later.

The game is fairly generous with its pickle drop rate so you'll have a ton of weapons and accessories by the end of level five, six or seven. Play with all the weapons and use the ones that you like best. You rarely get your ideal weapon configuration every game so you might as well be familiar with all of them. And don't start merging the guns until you have a full roster. Six lesser weapons do much more damage than three upgraded weapons. The most important part of combat is movement. Shoot your shots, then find the next safe place to attack next. Use the roll button to escape danger or put distance between you and the mob. Survival/safety is more important than your kill count. Try to avoid getting swarmed by keeping active and moving.

I'm a pickel wizard of fire soo I like the burning effect helps lean up the big guys also astetically soo shiny.

Once you start unlocking mutations, the guns with high fire rate become the best ultimately. Headshot mutation can instantly kill off any monster with a single shot, combined gaining health back on crits mutation translates to....more shots fired = better.

I tried an all melee build recently and could not beat the level 5 boss lol