According to Neil Armstrong, Apollo 8 was the most significant space mission!
It was first space mission where humans left orbit. 200 000 miles to moon, cf 600 miles! Russians thought to be 4 months away from moon mission, second Saturn rocket test failed (still most powerful rocket ever), 3 deaths in Apollo 1 accident, no lunar module yet. But decided to make 4 month target anyway! Lunar module still in development, so no back up motor.
First time anyone saw whole Earth disc (Earthrise photo from here). First time dark side of moon seen.