I think a good case could be made that he is the single most important actor in Star Trek history. At least, when you are looking at the franchise itself, as a whole. You could argue others within the confines of individual stories, series, and movies, and certainly for their work outside Trek, but for the franchise? DeForest Kelley was the most important.
He was the one that showed up to pass the torch from the TOS crew to the Next Generation crew, and everything subsequent follows from that. He was the one to tell us, in no uncertain terms, that this was still Star Trek. This was the universe, the Federation, and Starfleet of James T Kirk and Leonard McCoy, just a few decades later. He only had a few minutes in The Next Generation, but those few minutes were important to the overall sense of continuity.
You *could* argue Leonard Nimoy, since he also ties Abrams Trek to TOS and TNG, but he didn’t kick off TNG with a sense of continuity. Kelley did.