A launch mishap led to the best experimental confirmation of gravitational redshift.
Huge thanks to
Dr. Pacome Delva: https://ve42.co/pacome
Dr. Sven Herrmann: https://ve42.co/sven
Gravitational Redshift Test Using Eccentric Galileo Satellites: https://ve42.co/GRtest
Disclaimer: It is arguable what is THE best test of general relativity because there are different ways to test the theory. This is the best confirmation of gravitational redshift, which is one of the three original tests proposed by Einstein.