Answer: Back to the Future II

Carl Sagan was a huge fan of the sci-fi genre and particularly fond of movies that tackled difficult to navigate material such as time travel. It was in that mindset that he heaped praise upon the Back to the Future series, especially Back to the Future II.

Sagan told Robert Zemeckis and Bob Gale, director/writer and co-writer of the films, that Back to the Future II was the most accurate movie ever based on the science of time travel and did an exceptional job of handling the problem of the multiple time lines created by multi-trip time travel.

Bonus Trivia: Sagan’s name makes an appearance in the Back to Future video game. After finding himself in 1931, Doc adopts the alias Carl Sagan.