In your opinion, what is the best gui lib?

Blitz3D Forums/Blitz3D Userlibs/In your opinion, what is the best gui lib?

I'm hoping on doing a game like the sims... (very different... but that's the closest I can think of to explain how I want it set up) What (in your honest opinion) would be the best GUI library to use?
This is keeping in mind:
How fast is it to learn to use
How usable is it (user friendly and flexible)
How easy is it to include into a game?
And is it freeware? (prefured very much)

Some examples are: BlitzUI, AlphaUI & Xlt-gui
Please help... I'd be very thankful :)