My own favorites, after years at this, are the gobies and blennies. These are the little fish that LIKE to be within a few inches of a sandy bottom, and in and out among rocks, exactly the environment we have in a tank. They don't mind light, most aren't spooked by being watched, and they have droll faces and funny manners. They rarely do more than puff and bluster at each other over territory [the average goby territory is about 6-8 inches of bottom or rock base]. They don't bother your invertebrates [except copepods] and even the shyest [the highfins] become much more in-the-open once they know your presence can mean food. The watchmen? they watch things, mostly you, and they're kings of the tank, just ask them.
The classes of fish I list here are ich-resistent, don't get brook, I've never seen one with velvet or dropsy, and in general can live off the tank's micro inhabitants or algae if you have to leave your tank for a week. Once acclimated to your tank, they're practically bulletproof. They cannot, however, be mixed with aggressive fish. Tour the gobies and blennies, dartfish and dragonets on Foster/Smith for a good overview. Max size is 2-6", depending on species. The difficults in the group are the dartfish, which may jump if frightened, and the dragonets, which tend to eat only copepods off the rocks, but they all share the no-ich, self-feeding, low maintenance profile.