Another vote for Armor by Steakley.
If you could call it sci-fi, Vampire$ by Steakley would be my number 1. John Carpenter did a gigantic hack job to Vampire$ when he made the movie. IMHO the book is a thousand times better. Leaving out Felix was a sin!
Other sci-fi I like in no particular order:
The Starfisher Trilogy by Cook, got me started on sci-fi, still very cool.
Dune, of course...
The Mote in God's Eye.
Most of the David Drake stuff, I love his military sci-fi. Particularly Hammers Slammers, brutal, emotional, insight into the minds of ordinary people turned into killers by war.
The Casca series by Barry Sadler.