ExtJS is the one we actually use in our products...the widgets are just too awesome to skip. I'm not entirely on-board with the way things work. It's a very OO style project, and feels kinda like working with Swing or some other Java toolkit. A bit verbose at times. But, the library has so much coverage, it'd take us an extra six months to a year to develop those bits and pieces using jQuery, which I find more fun and natural to use.
YUI is a close runner up, and if I were starting today I might go with it. The widgets are fantastic, the documentation is fantastic, and the community is large. It also feels more like JavaScript than ExtJS.
As jQuery UI develops it'll probably become a contender in that space, as well. But, for now, I'm just adding it to our websites in a few places where it is useful--that's the other negative about ExtJS: It's a "drink the Kool-aid" kind of library. I've had a hard time integrating it in little bits and pieces, and it's turned out that everything is going to have to be converted to use ExtJS for it to all work together.
YUI is a close runner up, and if I were starting today I might go with it. The widgets are fantastic, the documentation is fantastic, and the community is large. It also feels more like JavaScript than ExtJS.
As jQuery UI develops it'll probably become a contender in that space, as well. But, for now, I'm just adding it to our websites in a few places where it is useful--that's the other negative about ExtJS: It's a "drink the Kool-aid" kind of library. I've had a hard time integrating it in little bits and pieces, and it's turned out that everything is going to have to be converted to use ExtJS for it to all work together.