Answer: Stephen Hawking
In the various incarnations of Star Trek numerous personalities have appeared over the years: Abraham Lincoln, Mark Twain, Genghis Khan, and Albert Einstein, just to name a few. Of all the famous people of our time and before, who appeared on Star Trek there was only one person who had the pleasure of playing their identity in real life: Stephen Hawking.
Hawking acted as a holographic depiction of himself in the sixth season of Star Trek: The Next Generation . In episode 26, "Descent," he appears in the holodeck, along with actors who show Isaac Newton and Albert Einstein, who hosted a round of poker with the ship-androids Data.
The crew and actors lovingly recall Hawking's visit. During a tour of the set Hawking was shown the engine room with the warp core of the ship. He paused to type something into his speech synthesizer and joked, "I'm working on it."
Courtesy of Paramount.