Biggest impact on me in Sci-Fi/Fantasy books were:
The Hitchhiker's Guide To The Galaxy 5 book Trilogy! (And the original Radio Show that started it all)
Some of the greatest, wittiest writing ever...while still being a great adventure!
The Stainless Steel Rat Series (12 books). I LOVED this series!
It has flirted with the big screen, but thankfully has not yet been made (incorrectly).
A young James Bond-ian Black Hat turned White Hat hero in space. Hilarious and inventive.
Jan De Bont (Speed, Twister) bought the rights in 2001, and came close to making it. I read the treatment, and it was typical Hollywood- They used the name, but changed just about everything else. Thank god it got scrapped.
Rendezvous With Rama Series
Of all the Arthur C. Clark books, why these? I dunno. They just fired my imagination.