Bukkit alternative?

I’m kinda creating my own Bukkit alternative, mostly because I’m bored and really wanted to ask some questions before I begin…

For Bukkit, Sponge, Spigot, TacoSpigot, Whatever developers alike:

What is the "best" part of your preferred API and why? 

What would be the most essential parts of every server plugin API?

Keep in mind I’m only asking developers, because the only response I’d get from users would be “all the plugins” and that wouldn’t make sense.