<p>What is the the best JavaScript compressor available? I'm looking for a tool that:</p>

<ul>
<li>is easy to use</li>
<li>has a high compression rate</li>
<li>Produce reliable end results (doesn't mess up the code)</li>
</ul>

YUI Compressor
is the way to go. It has a great compression rate, is well tested and is in use among many top sites, and, well, personally recommended by me.
I've used it for my projects without a single JavaScript error or hiccup.  And it has nice documentation.
I've never used its CSS compression capabilities, but they exist as well.
CSS compression works just as well.
Note: Although Dean Edwards's /
packer
/ achieves a better compression rate than YUI Compressor, I ran into a few JavaScript errors when using it.