Note: I work for Sencha (who makes Ext JS). I am also the creative director, so I would agree, we look the best :)
Now, "best" is relative, depending on your goals. Both of our frameworks, Ext JS and Sencha Touch, include every bit you need to make an app: Layout system, UI widgets, MVC, data binding, templates, I could go on. We also include a build system, so you can only use the pieces you need, so, even with all that we offer, your app is not "bloated."
That said, there are pros and cons to modularity vs. a unified system. There are some great microlibs out there, and independent devs are in an exciting stage right now, being able to mix and match frameworks as they see fit. From a team-based or enterprise perspective, though, this is not ideal: You are working with various APIs, certain frameworks can have duplicate code (ie. there's overlap in what jQuery and Underscore provide), and you're relying on individual devs or the open source community for fixes and enhancements. It's also a bit of a testing nightmare, as you need to re-test every time you update an individual component.
Also, we have a JavaScript-based API to creating things with our framework, which is quite different to the progressive enhancement method of other frameworks. The benefit to this, again, is performance, and a better architecture for an application. On the downside, this can be harder for novice web developers (those who focus on HTML/CSS instead of JavaScript).
At any rate, hope that helps some in making your decision. Let me know if you have any other questions-