Batman. I think Batman and The Doctor seem to be the fictional characters that will last for generations long after us, two characters we never get bored of no matter how many different iterations there are.
I could see Dr Who being relaunched one day though, and the character being dry docked for several years. It’s a little continuity heavy these days after the last few years and some of the villains are really used up, even though fans won’t let them go. It’s a bit like comics in that regard, so I could see a call in maybe a decade to relaunch Ultimate Dr Who. I think the BBC might have realized they’re in danger of overdoing it with Who (it felt that way with David Tennant) and so I can see them limiting episodes in response to keep him special. Like anything great, the secret to making it last is small doses rather than too much of a good thing.