Any Foxface. I absolutely adore my bicolor. Before we had a One Spot and that was a wonderful fish too. They are super healthy, really personable, and fun to watch. Plus they destroy algae as well as tangs do.
Bpb - where can you find any fish for $2May seem a silly answer, but If you can manage to get them without uronemia, a group of standard issue $2 green chromis are still one of my absolute favorite must-have fish. Especially if you can manage a decent sized shoal of them. Not much utility in them but they’re fun to watch