Of the "keepable" butterflies, there are a few you could consider. While not the flashiest, the Kleins is a very sturdy fish. Doesn't get too big and has some nice subtle colors. Relatively peaceful too.
Racoons are okay, but they can be pushy. I have a blackback going on 6 years. Lovely fish with a good disposition and a nice all around pick for a butterfly.
If you have some money to spend, look at some of the members from the sub genus Roa; Tinkers, Burgess, Declivis or Mitratus. Probably some of the hardiest bf's available but pricey.
If the tank is large enough, well established with consistantly good parameters, has a lot of live rock and no overly aggressive tankmates, you should be able to maintain a bf.
edit; if your profile is correct and you have a 55 with a yellow tang, I wouldn't add a butterfly. The tank size is marginal for one but moreoever I think the tang would punish a newly added bf, and they don't handle abuse/stress well.