Not saying it's "the best", but I've been using GWT-Ext at work lately. It's got some pros and cons:
Pros:
- Relatively easy to setup and use
- Decent sized community behind it
- LOTS of examples online, all of which have accompanying code
- Good selection of widgets that cover a wide range of functionality
Cons:
- Makes assumptions about your data's format and structure that you may not be able to accommodate
- Library is just a JSNI wrapper, which makes it harder to debug and extend
- Future support and development on the library is uncertain
- Some annoying cross-browser issues still exist (especially with layouts)
- Search function on their forums is essentially useless. (This is becoming a big pet peeve of mine >_<)
