Best Java obfuscator? [closed]
First, you really need to keep in mind that it's never impossible to reverse-engineer something. Everything is hackable. A smart developer using a smart IDE can already get far enough.
Well, you can find here a list. ProGuard is pretty good. I've used it myself, but only to "minify" Java code.
Check out my article Protect Your Java Code - Through Obfuscators And Beyond [Archived] for a discussion of obfuscation vs three other ways to make the reverse engineering of your apps more expensive, and a collection of links to tools and further reading materials.