Dendrobates leucomelas seem to be very good beginners' frogs. They are attractive, social, easily bred, and don't have any specialized requirements unique to them (like pumilio need bromeliads and some tincs need flowing water).
Other very good frogs for beginners are tincs (including azureus), most Phyllobates species, and auratus.
I'd go with the leucomelas myself, but tincs and terribilis are close rivals for the title of "best beginner frog".