" What is the best" question is natural (we all search such answer), yet a flawed one...
Except obvious cases (eg. here if Interfascia seems not enough for your needs), the answer usually depends on your needs, your tastes, etc.
Look at features: what library is answering the most your needs?
Look at screenshots: what interface is the most appealing for you?
Look at documentation: which one seems the most clear for you?
Look at existing sketches using these libraries (from examples to OpenProcessing site): what seems the easier / most natural to code for you? What is the most flexible?

" I find it strange there is no UI provided with the core libraries"
Some reasons:
- Processing is a rather low level graphics program, a UI is quite high level, using objects, interfaces, etc., concepts far from the design of simplicity of our beloved software.
- Designing a GUI library is hard, time consuming, and probably cannot please everybody (as seen above), so there is little benefit to provide one...

Hi phi.lho, thanks for your reply.