Pink streak wrasse for me, or even a pair. This is mainly for pest control. Sixline are also great and I have one in my frag tank on its own with some snails to help with algae.
I do have a yellow spot scorpionfish in what was a frag tank, so consider things that you wouldn't want in your reef tank (the scorpion I have would be fine in a normal reef but it's hard to find where it is half the time and I don't want to get spiked). I say it was a frag tank but it's basically just a massive stylophora coral now in which the scorpion lives quite happily.