Carl Sagan, hands down. I read The Demon-Haunted World when I was in middle school, and it had (and continues to have) a profound impact on me. The chapter that continues to stick out in my mind is on hypnotherapy and satanic panic/child sexual abuse allegations, as well as the related phenomenon, this one very big in the 1990s at the time, of alien abduction. In law school I took a course on expert evidence and was astonished at some of the rulings admitting hypnotherapy evidence. Thankfully, there were some skeptical appellate courts that acted as gatekeepers and reversed convictions based on what was clearly pseudoscience in hindsight, but there were just as many failures, many that probably escaped notice.
I also loved reading Isaac Asimov, Arthur C. Clarke and David Brin around this time. Their work did quite a bit to popularize science, expressly in some cases and implicitly in others.
Today, I would say that one of my favorites is Neil Degrasse Tyson. I also appreciate Richard Dawkins.