I guess if you're patient, you can find something truly unique. There's are definitely a number of tangs are just not imported. A couple months ago, a LFS got in two Naso brevirostris
which I've never, ever seen or heard about in the hobby. They were absolutely striking, a darker brown/black body with a white stripes just aft of the pectoral fins, and the brighest white tail I've ever seen. The problem is that these guys were already 12" and anything short of 500gals would be cruel....
How big of tank are we talking about? For anything six-feet and up, for an unusual but not one-in-a-million tang, the Mustard, aka Jewel Tang, Acanthurus guttatus
gets my vote.