Fiendishly clever by incorporating the means to recast the lead into the very premise of the show, Doctor Who remains the longest-running science fiction program in, well, any kind of history. In fact, it celebrates its fiftieth anniversary this year.
Eleven actors have played The Doctor (with a twelfth on the way soon), all different incarnations of the same man thanks to the Time Lord's ability to regenerate. The shows over time have spanned virtually any kind of tale that can be told within science-fiction – and beyond.
There's action, humor and a fair amount of wrenching drama (don't believe me? Witness the end of David Tennant's remarkable run).
Granted, the good Doctor left the air for a time in the '90s, but it's safe to say that he's made more than a firm comeback.
At 30+ seasons and counting, Doctor Who must be the top sci-fi show ever.