I've used YUI and found its slider, color picker and tab controls to be really useful. I've also used Plotkit for creating web charts. Plotkit is really nice. Its built on top of Mochikit, which I thought was dead (it hadn't been updated in over a year when I was playing around with it), however, I just visited their website and it looks like they're updating it again.
I've been thinking about learning JQuery, but I keep hearing good things about Prototype too. At some point I'm going to sit down and try to learn one of them in depth. Though the idea of writing to learn to write a framework from scratch sounds pretty interesting too.
Anyway, I can recommend YUI and Plotkit, both of them are very nice, however, I'm not sure how they compare to all of the others. When I decided to go with YUI, I made the decision because I wanted to use a sliderbar in something I was doing and YUI had the best looking sliderbar of any of the frameworks I looked at.