For me, it was The Songs of Distant Earth and Odyssey 3 that have solidified certain certain political and religious beliefs. Also Rendez Vous with Rama. My copy of 2010 is battered and in some places torn but it remains one of my most treasured books, alog with Carl Sagain's The Demon-Haunted World, a book that should be required reading.
Semper Fi, Sir Arthur C. Clarke.