In your opinion, what is the best gui lib?

Yes, Lite is free version. FUI is a "pixel perfect" 3D GUI. It doesn't use any realtime 2D functions, so it doesn't have the slowdown that you get when you have 2D on 3D. The GUI is very complete, lots of gadgets and a couple of optional Win32 Dialogs.

You can take a look:
http://www.blitzbasic.com/toolbox/toolbox.php?tool=10