I`d say the king of bad weapons is the Royal Blade followed by Burst Blade and Poseidon Cannon.
I didn`t like the Aurum Palm very much either because of its lack of homing, though I`ve only tried it once (SCF version) so it might have some potential with its great damage output, range and charge time.
The Boom Orbitars and Ball Cannon are good weapons which can easily OHKO with fast shots so they definitely can`t compete for the title of least effective weapon.
The Cancer Claws are powerful and fast, their only problem is that they`re outclassed by Brawler and Raptor Claws. However if these two were banned, Cancer Claws would be my next choice for melee claws.
Bomber Arm (DCS version) is actually very good despite its stats. It plays similarly to a Magnus Club using short ranged DCS with great shot speed, homing, and charge time. However it trades the ability to OHKO for a 2HKO with higher running speed, shot range and much less recoil after a DCS. It`s definitely in my top 10 and maybe even top 5 favorite weapons in the game.
royal i run full range game with sr+3 and 10 range stars, the homing is good enough that i don't think you have to use homing boost
and then my burst i run kbd+4 and eva+4 with i believe 5 r stars, using the cancel for approach, the eva to dodge the harder to evade weapons and the kbd lets me attack sooner if i do get hit.
Do you use the DCS or DCF? I`ll try to make a good one to see whether I`m not missing something because so far they seem horribly outclassed by the Aurum Blade.
After searching through my weapons I realized that I actually have a pretty decent Burst Blade (V297, 6R/0M, Shot Defense +3, RS+4, DCS+4, IPAB-4) which is waiting to become another Bomber Arm and even though it has 10 ranged stars it needs to attack from 20m or less in order to 2HKO. How do you get kills with yours if you only have 5R stars?