For Primary guns, I would pick the adamant laser cannon. Good capacity and high rate of fire. Perfect your aim and you have a rapid-firing gun that can hit even at long ranges. Yes,it kills faster than the mega gun.
Champion Mercenaries take the backup section for the same reasons. Hard-hitting, good capacity, and can even hit at long ranges with skill.
Melee weapons fall down to the Jet Tenderizer really quick. That one-hit kill out performs the storm hammer by a huge margin, and the good and evil knives are a little.... gimmicky for me.
For special weapons, who doesn't like the Poseidon's trident? It's a one-hit kill with a good fire rate to back it up. The Vacuumizer is a close second for me, though due to its laser power.
Sniper guns.... let's pretend the anti-champion rifle doesnt exist xD? Jk, but besides that, there is a tie for me here: the sakura and the shadow spell. The Sakura can kill SURPRISINGLY quickly, especially when quick-tapping the fire button. The shadow spell can one-shot on a direct hit, though you have to be presice.
And finally, the renegade fills in the heavy slot. I don't see a lot of people use it, but boooy, if only they knew. Each rocket is a one-hit kill, its splash damage is large and allows for group shots, and the fire rate mows down everyone. That's a potential 12-15 kills per clip, more than I can estimate from the solar cannon!
What are your most powerful weapons?