The hottest JS framework, according to Google Trends
google.com
google.com
A bit later, when I started doing some more front-end work, I found jQuery, and I dropped Dojo entirely because I found I was much more effective with it (a little bit / [lim -> 0] is...). Since then, my anecdotal experience has been that jQuery is the first-choice JS framework among the circle I regularly interact with.
All this is to say: I feel like my preferences toward JS frameworks as a relatively new developer are quite similar to this chart.
http://hluujavablog.blogspot.com/2005/01/framework-vs-librar...
eg "jquery plugin to add 2 numbers!!!!"
I don't think that really tells you much about quality though.
other then that, YUI is much more professional to use.
most web searchs for jquery keywords you got a zillion blogs with 'plugins' on how to do stupid little things. yui has a nice documentation and is consistent even when you account community created widgets.
yui can be a little too OO centered, i recognize.
and on the end. diversity is always awesome!
I've used both quite a bit and I've found the YUI documentation to be frustrating to navigate and overly complex. YUI reminds me of the standard Java libraries in all the wrong ways. jquery has fantastically straight-forward, simple documentation.
About the documentation. it's a reference guide. it's like reading a well formated source code. and the search feature there is top notch. I like it. That's the main advantage to me. jquery tries to patronize me sometimes.