I still love powder blue tangs. They are also one of the smallest tangs, but one of the more aggressive, especially in smallish confines.
Other favorites: powder brown tangs, red sea sailfins, naso literatus. There are tremendous differences in the coloration of fish; not so much the patterns or anything, but intensity and whatnot. One of the most beautiful fish I've seen in the past year was a powder brown at Skipton's a while back.
Naso's get big, big, big. I'd say your best bet is an Acanthurus or certain Zebrasoma. Either way, I'd recommend adding them last or close to last as many of those guys tend to bully new fish, especially in a tank that size.