LuckPerms is an advanced permissions implementation aiming to be a flexible, performant and reliable alternative to other permission systems, providing all the features you could ever possibly need in a permissions plugin, along with an extensive and well documented API for developers.
_Features checked on 23rd Aug 2016. If you find any inaccuracies, please do let me know. I tried to be as fair as possible, and copy all major features from the respective plugin pages._