MemberPress is the easiest to use but its not free and has its limitations. But 'Paid Member Subscriptions' is a nice little plug in and you can start with a free trial to see if you like it and play with the features. Personally I think Wild Apricot is the best option, its cost effective and has most of the feature you will ever need. Its not a plug in though, its a stand alone butttt it incorporates very nicely with word press. Hope this helps.