What is 'best' depends on your exact requirements. Just what a RAD IDE should povide is rather vague - see Rapid Application Development. If you want a good general-purpose Java IDE with a wide range of plugins, and you don't mind paying for it, I would recommend IntelliJ IDEA. NetBeans and Eclipse are good, but many professional developers think IDEA is better.
When our company wanted to settle on a standard Java IDE, we organised a team of developers to try each IDE for a period and compare them. IDEA was a clear winner - for our requirements.
The tools we use have a profound (and devious!) influence on our thinking habits, and, therefore, on our thinking abilities...
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