2001: A Space Odyssey
Read this if you have an interest in the origins of the genre.
Traditionally this book is my favorite SciFi novel. Other times I imagine I might rather read Fountains of Paradise or Rama. But, the fact that this is a series with returning themes and characters (and a series written by Clarke) bumps this up to a higher spot for me.
+ Very Clarke; little character development, tense moments, great hard scifi.
- The aliens feel more supernatural than advanced.