There have been many long lived shows on tv, each having characters you could not imagine being played by anyone else. James Arness in Gunsmoke or Mariska Hagitay in Law & Order. Kelsey Grammer played Dr. Crane for 20 years in two different series. These actors are nothing short of iconic in longevity and their character portrayals.
The actor who best embodies this for me is David Suchet as Poirot. After 25 years, this series is coming to an end with the final five episodes now airing on public tv. Yes, other actors have played Poirot over the years, but no one captured his essence in the way that Suchet clearly has.
As always, please post a picture for those of us who may not know your actor.