That is a question I asked myself, too. For now I'd say arrancars don't have a bankai or something similar - actually they don't have a real shikai either:
Edorad stated that the zanpakutou of an arrancar is the core of their power and once released they reach their full power and take on their true forms. He also said that arrancars zanpakutous are the opposite of a shinigami's sword (whatever that means, I don't understand it...), therefore you probably cannot compare the release of an arrancars sword with the shikai of a shinigami.
And if they are in their true form after the release there shouldn't be a higher powerlevel for them after that - so no bankai or such a thing. Maybe...
