* Longest running show in total: This would likely be "The Simpsons", which started airing in 1989 and is still ongoing. It's been in reruns for decades, and continues to air in syndication.
* Longest running show in reruns currently: This might be "Gunsmoke" which aired from 1955-1975, making it over 60 years old. While it's not currently airing on a major network, it's still available on streaming services and classic TV channels.
* Longest running show in reruns on a specific network: This depends on the network. For example, "Law & Order: Special Victims Unit" (2000-present) is likely the longest running show currently airing in reruns on NBC.
Ultimately, it depends on your definition of "longest running" and what you consider to be a "rerun" (e.g., streaming vs. network TV).
Let me know if you have a specific criterion in mind, and I can try to give you a more precise answer!