Personally I use LuckPerms, and apart from it being the highest rated resource on spigotmc, I think it boasts the most features, and the easiest setup. Lately Luck (The creator) pushed an update which revamped web editing, which allows you to edit perms for groups and users in your browser with a neat GUI. Really Recommend it. And GroupManager is dead. It sucks, and relies on Essentials to run, which itself is a dead weight.