Psion sucks for two reasons. 1: it's a made-up word that means nothing outside of D&D, and 2: it's homophonous with an unrelated real-life word that comes up a lot in fantasy. The first time someone suggested that I play a psion, I immediately wondered, "Scion of what?"
Psionicist is just too awkward to pronounce. Also, it's a made-up word that means nothing outside of D&D.
Mystic has many connotations, none of which have anything to do with psionics. Not a great fit.
Psychic seems perfect to me. It's a real word, it's easy to pronounce, and it clearly expresses the concept that it represents.