Literally psychotic? Kefka, hojo, and Sephiroth would be the ones.
Kefka has no motivation beyond seeing what happens if he smurfs with stuff to the maximum degree
Hojo is willing and wanting to do anything out of curiousity, especially if it's taboo
Sephiroth seems to have no will of his own, merely blindly following "mother"s assumed wishes, although really thats more sad than anything

Arguably psychotic would be Bartandelus, but his real goal was freedom despite all the terrible things he did

The least Psychotic in my opinion would be Vayne, Palamecia, and Ultemecia. Maybe Kuja too.