Lake tangykian shell dwellers, a simple tank with sand and shells, plus very colorfull if you choose Lamprologus' ocellatus (Gold), this is garunteed to be your most active and likely colorfull as well as interactive setup for a 20 gallon
I don't see how anyone can answer this for you. What do you like? At least name a couple of types you have an interest in, and then people can help you decide if they are appropriate, and what tankmates would go well with them. Do you have any visions of the aquarium at all?