What's the best/easiest GUI Library for Ruby?

Ruby Shoes (by why) is intended to be a really simple GUI framework. I don't know how fully featured it is, though.

Some good code samples can be found in the tutorials.

Also, I think shoes powers hackety hack, a compelling programming learning environment for youngsters.

Here is a good resource for you:

http://en.wikibooks.org/wiki/Ruby_Programming/GUI_Toolkit_Modules

has links comparing basically all of them.

I started with FXRuby because it had a book.