Don't get danios first of all because they need more room to swim.
Keyholes and angels get too big for a hexagonal 15 gallon.
If you want a fish that'll stand out and do well in your tank, go for a gourami - a (male - brighter colors - colisa lalia) dwarf would work (but they can be fragile and are the smallest) or a banded gourami (colisa fasciata) or a thick-lipped gourami (colisa labiosa) or a pearl gourami (trichogaster leeri) or possibly an opaline, three-spot or golden gourami (trichogaster trichopterus) provided the tankmates are suitable.
I would personaly go for either colisa fasciata or colisa labiosa in this instance as both get to about 3", will not be aggressive towards tank-mates, are relatively hardy fish and are very colorful. You could combine this fish with a pair of rams, 6 or so neons and a couple of small loaches such as botia histrionica (gold zebra loach) which gets to 2.5" or maybe a couple of otos which would also take care of algae and grow to 2". Pygmy cory cats also stay small but they aren't realy bottom-dwelling. A small plec you could get in place of a couple of loaches/otos would be a bristlenose as these get to about 4".