That's a thoroughly intriguing question. I was going to say Reddish South to Denton, but lots of enthusiasts probably buy stuff like that even if they never travel. Wouldn't surprise me if it was two arbitrary points on a commuter line where almost everyone is going into the big city at the end. I'm going to venture that there isn't much demand from, say, the small, pleasant village of Broadbottom to the rathole that is Hattersley - but then maybe some put up with it as they work in the big Tesco there. Or then it could be something like the two hardly-used stations in the middle of the Cumbrian Coast.
Actually, even more unlikely on the Hadfield line - I would venture the opinion that there were perhaps zero tickets sold from Hattersley to Ardwick? (There is only a westbound service from Ardwick at present, which is intriguing).
I would imagine there are at least some where the sales are actually zero even though the fare exists, so there will be quite a number of answers to this question.