I want to develop an application with simple Quiz-like games (think of a question , answer, a timer, high-scores/ranking, not much more)
For that I want to use Rails and some javascript library that let me have some modern effects on the GUI (being a GUI the least you can ask is for an atractive interface)
I first considered ExtJS but now I think is too serious for this.
Also there are millions of extensions for prototype(http://stickmanlabs.com/lightwindow/ http://underwhelm.net/pinwheel/ etc) that looks more casual but dont know if they will play well with rails
So what is in general the best Javascript gui/widgets library out there when the backend is going to be Rails?
And in my particular case what would you recommend?
Scriptaculous will be your best choice. Other libraries will conflict with Prototype, if it is present. If you are creating one simple interface with the features you mentioned, you can probably get away just using Prototype/Scriptaculous and a few plugins. JQuery is great, but the $ conflicts with the Prototype override, causing it to be more difficult to work with.
If you need help with prototype, I found that http://prototypejs.org/learn is useful.