there is no best 3d library.
flare3d is, i believe, the most complete library with the best documentation. but it's not open source. if you have a problem, you can't check a class's innards.
away3d is open source and has been around a long time (in one form or another) so there are lots of tutorials/help. but much of that is from older versions that may mislead you.
alternativa 3d requires you add a link to their development website and, as of 1 year ago, was the least complete library.
addendum: here's a reasoned discussion from 1 year ago: http://away3d.com/forum/viewthread/54/
