Someone mentioned the Convict surgeonfish (Acanthurus triostegus) as a mild mannered choice. I'd second this one. My experience was good, although it was one of two fish, the other being a Six-Lined Wrasse, in a 100 gallon tank. Got him around 1.5" diameter, grew rapidly to almost 4" or so within a year.
The Altantic surgeonfishs seem to be easy going too. If I recall right, Bob Fenner was mentioned several times in articles that the Ocean surgeon (Acanthurus bahianus) and Doctorfish tang, (Acanthurus chirurgus) are quite well behaved. I have no experience with either of these fish.
I will say that these 3 choices aren't the most colourful and it does seem odd to recommend ones from the genus Acanthurus. Again, the Convict surgeon is the only one I can comment on. What do other Convict surgeon owners, past and present, have to add?