Probably not the longest because I have shit memory and I have a long list old media I want to consume someday, but:
Peaky Blinders.

I've wanted to check it out since the first season aired back in... 2013 omg. And i still haven't ahaha
It's on netflix, too, so it's not like it's a difficult series to start to watch.
But it's like idk. There... I will probably love it... But... Mmmmh? I don't know. I'll end up watching it in 2033.