I have used graphite nuts in tremelo guitars along time ago, they slide best IMO. But you dont have a trem so no need for it, Tusk sounds best, maybe bone arguably a bit better but i cant hear a difference at least, well natural bone has the advantage that you can pick the best matching sound by dropping them on a hard surface, more of a thing for the real pros if you ask me, but probably they dont even bother either :lol:
I have a Brass nut in my 25/50 anni custom, it gives a bright and unusual deep sound, very 50's gibson custom like IMO, but im not 100% sure since i havent had any other nut in there so i cant compare before and after sound.