What was the first SpaceX rocket stage to successfully land and be reused?

The Falcon 9 first stage was the first SpaceX rocket stage to successfully land and be reused.

On March 30, 2017, SpaceX launched the SES-10 communications satellite using a previously flown Falcon 9 first stage. The booster landed after launch and later became the first orbital-class rocket stage to fly again for a second mission. This demonstrated the practical value of SpaceX’s reusable-launch strategy.

The first successful landing of an orbital rocket booster had occurred earlier, on December 21, 2015, during the launch of Orbcomm-2. That booster was not yet reused, so the 2017 SES-10 mission was the key reuse milestone.

The Falcon 9 first stage performs the high-energy portion of ascent, then separates and uses engines, grid fins, and landing legs to return to Earth. The upper stage continues carrying the payload into orbit.