There are so many Role playing games on andriod so the best might depends on individual choice. As we know one man's meat is another man's poison. But definitely Role playing games are really interesting genre of games where one can really spend time crafting characters playing some storyline or generally just enjoying the games.
There are so many of role playing games like the Bards tale, beamdogs, doom and destiny, mage and minions, exiled kingdom, evoland, fire emblem heroes etc. For me my best has always been the Mage and Minions. This is a game that is free for all and one can as well play it offline. It a game with diverse story lines and concept which one will be battling with tuff like robots, aliens, skeleton and beast. It an interesting game.