"Best" is the most subjective word in the English language, so the answer, of course, is "it depends".

Both are good frameworks, and each have their pros and cons. jQuery is far more popular overall though, which means that you are going to find a lot more tutorials, books, plugins, etc. for jQuery than with Mootools.

If you want to learn a framework in order to be more "employable", then go with jQuery, simply because it's the most popular JS framework out there, by far. But if this is for your own personal projects, then why not try both frameworks, and see which one you like best?

Here's a pretty in-depth article that compares the two...

jQuery vs MooTools: Choosing Between Two Great JavaScript Frameworks

If you're starting I would recommend jQuery, why? because you'll find tons of resources, documentation, articles...jQuery is a commons thing around....

jQuery vs MooTools: Choosing Between Two Great JavaScript Frameworks

I personally prefer Jquery