Best is a matter of opinion. It depends on what your trying to accomplish. The Graal Wiki (Scripting Bible) seems to have alot of usefull information. FP4 and Twinny both have outstanding tutorials for beginners. I will link to a few to help you out.
http://www.graal.net/index.php/Main_Page
http://public.zodiacdev.com/index.php?title=Fowlplay4
http://gs2.graalstudios.com/
If you go to the first link, the links to the rest are at the bottom. The first link probably contains the most information tho navigation thru it seems to be quite difficult. I know I have seen pages on there and tried to search and searching says no pages found but I know for a fact they exist.
If your main goal is for a beginner to learn GS2 I HIGHLY recommend going thru FP4's first as it's probably the best written one I know of.