Obviously barring child actors. One comment (https://www.quora.com/What-television-actor-aged-the-quickest-across-the-course-of-multiple-seasons) that I read suggested David Soul's drastic aging over the course of Starsky & Hutch's four year run.

My candidate is Andy Griffith over the course of Matlock, 1986-1995. His appearance changed significantly, although I realize the series had a long run.

Probably Gil Gerard from Buck Rogers.