Dhalgren by Samuel R. Delany is my #1 SF novel. Unfortunately it's not available in ebook form.
Foundation Trilogy - Asimov
Ringworld - Larry Niven
This is a great list of classic SF Novels:
http://home.austarnet.com.au/petersy...oks_rank1.html
and short stories:
http://home.austarnet.com.au/petersy...t_stories.html
I just re-read:
A Rose For Ecclesiastes Roger Zelazny
available here:
https://www.msu.edu/user/carterca/rose.htm
and
Those Who Walk Away from Omelas - Ursala K. Leguin
available here:
http://harelbarzilai.org/words/omelas.txt
both are excellent.