The Wright Flyer
This is the very first "plane" ever, designed by the famous Wright Brothers. According to the Smithsonian, it was "the first powered, heavier-than-air machine to achieve controlled, sustained flight with a pilot aboard." Obviously, it was not meant for passengers.
It could only be flown for less than a minute and the pilot had very little control over it. The Wright Flyer had about four flights before it was retired on December 17th, 1903. In comparison to how far flight has come, the achievement seems small but it was pivotal in aviation.