WePay is great for anything based around a group. They have a really nice API - I built http://www.everribbon.com with it
Amazon FPS is also really great if you want to do anything complicated. Unfortunately this means that the API is also very complicated and a bit hard to dive into.
Shoot me an email if you want to ask me questions about either one.
Chargify made the integration ridiculously easy. Initially I wanted to go with BrainTree but the deal we got from TrustCommerce was more favorable to us since we do not yet have much traction.
Would love to hear from the others.