This is a hard question! I want to say "just grab any book!" because it's that good.
Look to Windward: An optimistic and at times funny romp focused on "the culture"
The Player of Games: about political freedom, masculinity, power.
The Hydrogen Sonata: haunting, about civilizations dying.
Excession: Personal favorite, though it gets very abstract. About pacifism & what *real* super intelligent AIs might do.
Use of Weapons: Don't read this one first. It's confusing but good.