I liked "Orphans of the Sky"
It's been years since I read it so I really can't say it is his best but it is definitely one of my favorites
Welllll... Glory Road isn't strictly fantasy, nor is it strictly science fiction. It has a good many elements of both. And no, it wasn't the only such he wrote. Waldo & Magic, Inc. is a book made up of two fantasy novellas, while most of the things in The Unpleasant Profession of Jonathan Hoag (a.k.a 6XH) would also classify to some degree.... For that matter, a fair amount of what happens in The Number of the Beast could well classify, while Job: A Comedy of Justice, certainly does....
Not Job. That one lacks all criteria to be viewed as anything other than fantasy -- especially as it is modeled on Cabell's Jurgen, which is one of the classics of the genre....
Which is interesting since i didnt know that about Job.
Maybe its an underrated book of his when its not mentioned as a fantasy book of RAH.