the hardiest butterflyfishes, ime, are the merten's, pearscale, punctato, henis, threadfin, raccoon, and saddlebacks. either of the first three would be fine in a 90. as already stated, make sure you see it eat at the store before you bring it home. they are also prone to bacterial infections from shipping. you can tell an infection by red spots usually around the mouth.
a vermiculated angel looks and behaves much like a butterfly, if you want to go that route instead.