The best API for plugin developer, with Economy kit, Database SQLite kit, BStats and Spigot Versioning The PDEK (Plugin Development Essentials Kit) is a API for plugins developers, yes, if you're a developer this plugin can help you in diverses way.
Economy Kit

This plugin has a Economy Kit (Only depents vault), that kit can:

Take the amount money of the player, this is very usefull for "if"s
Add money, this plugin can add money directly to player bank, independent of Economy plugin
Remove money, this plugin can add money directly to player bank, independent of Economy plugin
Set money, this plugin can set money of the player independing of independent of Economy plugin

this kit is very useful to make a price for things, like tell or sethome for example.

To see more about the Economy Kit, click here

A Database Kit:
This plugin have kits for database, very usefull to save diverses data, to see more about databases click here

You can make a unique database SQLite file for plugin, but in this file, is possible to have diverses table, for example: "Register Table" for a register/auth plugin, a "Score Table" for a plugin for Scores

To see more about the DataBase SQLite Kit, click here

Bstats Kit
This plugin have a Metrics.java ready for be used and not needs you downloading the class in the github or create a pom.xml dependence only to get the Metrics

More about BStats in: https://bstats.org/

The Metrics.java class: https://github.com/Bastian/bStats-Metrics/blob/single-file/bukkit/Metrics.java

To see more about this kit click here

Spigot Versioning Kit
And, for now, this Plugin-API have a Spigot Versioning ready to be used in the OnEnable function, only needs you just need that your plugin has a spigot resource with your own Spigot ID

To see more about this kit click here

You have interest for the PDEK Plugin-API? if yes you can see how to install/implement and use this in your own plugin leaving him more best and useful clicking here