Retard.

Eh, I use it for very stupid people. Obviously devoid of ableist intent.

I feel as though the context matters with this. For the genuinely evil and criminally unintelligent I would use the clinical “Mentally retarded”.

“Retard” and music (low volume) on buses are the controversial hills I’m willing to die on.

Pick better hills.
Maybe later, for now I have petty culturally unpopular positions that I will maintain. They are few but they are mine.