see http://mindprod.com/jgloss/obfuscator.html

I have no experience to lend, but the two most commonly mentioned names
I see are RetroGuard and DashO.

try this http://www.zelix.com/klassmaster/

I don't think there is the "best" obfuscating tool, for anyway, the code
will still decompilable. Now if you want to give a pain in the ..... to the
person who will decompile, there are two well-known free products:
- Retroguard
- Proguard

I personnally use proguard, as it looks to create smaller classes, and
optimize quite well unused code and classes ( I use it for J2ME
development).

--
JScoobyCed

half my students