Best weapon in DS3 is:
elite heavy frame
upper: telemetry spike + difraction torus = chaingun with at least 80 ammo
lower: plasma with default tip for knockback
electric ammo attachment can endlessly knockback enemies, unlike the acid or fire ones
another attachment - ammo or dmg
circuits dmg for chaingun and ammo and reload for plasma
I personally prefer the elite heavy frame with two military engines - top direct ejection field (creates a carbine) and bottom conic disperal (creates a shotgun). It doesn't matter which "effect" you take because if you just throw in a few damage/clip/rate of fire chips, nothing gets close enough.
i've made it early ingame, and other weapons i took along were lot worse, although still fun
I beat it already on the hard mode (don't play it on an easier difficulty or it's too easy) and am currently playing it on impossible with a friend. Co-op is MUCH more fun than single player.
another fun gun is dual rocket launcher with reload +++++ and explosion amplifier and explosion shield
I use safety guard (explosion shield) and ammo box. So you get your clip without reloading. I also use the cryo pneumatic torch - it's SO good against some of the later big baddies. I had used the chain gun with acid earlier though, as it really helps against the swarming dudes.
dual saw is also nice but a bit weak and too situational
Don't bother with dueling the saws. If you use the default tip, you can have it spinning in front of you while you fire your other barrel. I've been wanting to try the saw with an electrocution module, plus a stasis attachment - super slowing saw.