Hmm... almost everything I'd put on a list is already there, so the originals...
Cordellia's Honor by Lois Bujold (I guess this counts for two as it's Shards of Honor and Barrayar in one book, then feel free to kick one of the others out, because something that counts as a fun romp, a tear-jerker, and a romance just ought to make it on the list)
The Mote in God's Eye by Larry Niven and Jerry Pournelle (feel free to ignore the sequel)
Lucifer's Hammer by Larry Niven and Jerry Pournelle (because one of their books where a giant rock hits the earth ought to be on the list)
Foundation by Isaac Asimov
Ender's Game by Orson Scott Card