My votes for best sci-fi books have already been listed (God bless you guys), so here are some that I think are also awesome:
The Moon is a Harsh Mistress - Robert Heinlein
The Forever War - Joe Haldeman
Rendevous with Rama - Arther C Clark
The Moon is a Harsh Mistress
Oath of Fealty
Dragonflight
Excellent picks. The whole list could just be Heinlein books