Aviation Exam and Bristol are the 2 big ones I reckon. Most people I know use one of these.
AE has a better interface, a good app, extensive explanations and comment sections on each question.
Bristol has a worse interface and no app BUT I think the quality of the bank itself is a bit better as in less questions and more to the point. Also the explanations are more concise.
Anyway, anyone you pick you'll be probably fine.