Accept subscription payments, manage customer profiles, deliver digital content, integrate with your blog, forum, or CMS, send opt-in newsletters, run your own affiliate program everything is easy with aMember Pro. I dont know if its the best, but certainly lots of people are using it and features seems to be quite interesting and efficient.
Would be very handy for people selling digital goods like our beloved WSOs. So, if you are planning to start a WSO, you might wanna use it as it would provide a much efficient method to keep the subscriptions in control and upto to some level secure.

Features
-AS 3.0
-Only 40kB without embedded font
-FLV, F4V , MP4, MOV (H.264 encoded), Youtube and Vimeo support
-Minimalistic design
-Sliding transitions
-2 themes included
-Special characters supported
-More than 40 XML settings available
-Easy loading/deleting to/from your project