YUI Compressor was the consensus best tool for minimizing, but Closure seems like it could be better.
From the comparisons I've seen, Closure seems to be the clear winner in terms of minimizing file size. This article uses three popular JS libraries (jQuery, Prototype, MooTools) to compare compression between YUI Compressor and Closure Compiler: http://www.bloggingdeveloper.com/post/Closure-Compiler-vs-YUI-Compressor-Comparing-the-Javascript-Compression-Tools.aspx
Closure comes out in front in each test, particularly in its advanced mode, where it "minimizes code size about 20-25% more than YUI Compressor by providing nearly 60% compression."