The best IDE for java is different based on what you need, the two main ones that i see alot (and personally use) are:

1.) IntelliJ 

2.) Eclipse

IntelliJ is a very feature packed IDE with great addons and some great built in factors, Its only real downsides are its CPU and memory usage (There is also a paid version but the free one works just fine!)

Eclipse is a very light weight IDE where almost everything is installed via addons, making it super configurable

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TL;DR:

If you want a feature packed IDE go IntelliJ, if you want a light weight IDE go Eclipse