Some of my personal favorites ...

• The Gentle Giants of Ganymede (Giants Series) by James P. Hogan: How it revolutionizes our perceived history and place in the Solar System, and the whole idea of us finding an alien species and all they want to do is go home. Was made into a manga in Japan!
• Lucifer's Hammer by Larry Niven and Jerry Pournelle: A great take on surviving a global catastrophie. Especially enjoyed the scene at Cal Tech when they run outside.
• Fallen Angels by Larry Niven, Jerry Pournelle, and Michael Flynn: The concept of a distopian future and astronauts being caught 'on the ground' and just trying to return to their space station. Never imagined car heaters would play such an important part in a story! :p
• The Stainless Steel Rat (Series) by Harry Harrison: A great anti-hero series, lots of books in the series.
• Footfall by Larry Niven and Jerry Pournelle: Another we meet aliens series, well done.
• The Mote in Gods Eye by Larry Niven and Jerry Pournelle: Heh, those two like to revisit a subject from a completely different angle.
• Startide Rising (Uplift Series) by David Brin: Just loved the idea of talking dolphins and apes, etc.
• With the Lightnings (Lt. Leary / RCN #1) by David Drake: A great series of space combat, and duty to service, etc. Taken from the old non-scifi series of novels of adventures on the open seas. The space combat from a distance is an enjoyable different take than what you would see in Star Wars/Trek, etc.
• On Basilisk Station (Honor Harrington / Honorverse series) by David Weber: Another great series of novels based on naval military warfare and duty to one's people/service.