This classic Western series is best known for its endurance. It debuted on CBS during the first golden age of television and stayed there until the mid-1970s; its presence on TV spanned two decades and five presidential administrations. But Gunsmoke is also credited with bringing more depth and nuance to the genre. Although you might have to squint to see it, this is the pioneering predecessor to Deadwood and Westworld. 20 seasons, 518 hours.
Six seasons available on Amazon with a Starz subscription. Also available on DVD.