Apollo 10

On May 18, 1969, three men set out for the moon: Thomas P. Stafford, Eugene Cernan, and John W. Young. The goal was a "dress rehearsal" for a full-moon landing, including full staging of the craft (but stopping short of the surface.) The team came back on May 26, having orbited the moon without quite landing.

It wasn't the first crewed lunar orbit. That honor belongs to the Apollo 8 mission. But this was the mission that made sure NASA could pull off a full scale landing.