Dart frogs, definitely. Once you get the hang of culturing fruit flies, it's a very cheap and sustainable aspect of the frog hobby. Interaction and activity wise, Dendrobates tinctorius (of which "Dendrobates azureus" is now considered a race rather than a separate species) wins over any other frog I know. Phyllobates terribilis is a beautiful species, and incredibly bold, but they rarely move when not eating, so not very entertaining to the on-looker!