I've used Java Swing, Cocoa, Qt, Android. They are pretty good and were enough to build any kind of complex UI that I needed. I'm not sure what's the best one in the world, but generally I've found most GUI layout managers pretty good.

Even basic tools like those which were found in old Delphi, with anchoring components to the parent side, were sufficient for many UI-s, although some tasks like internalisation could case some pain.

In the end my opinion is that there should be a balance between usability and ease of implementation. Especially for technologies which are supposed to have many implementations.