Ask HN: What is the best cross platform user-interface toolkit?

Specifically user-interface toolkits that use the operating system's own windowing toolkit under the covers.

Also nice to know are things like:

* Does it stick to the platform's best practices or does it provide "the lowest common experience"?

* What platforms does it run on?

* What could it do better?

Basically looking for prior art and developers experience with them.