What line has the fewest services?

[bluejeans]

I think the Sapporo(?) - Wakkanai one would be right up there, along with the Kiryu-Mato line, and whichever one that was in the news for picking up that high school girl instead of closing.

[Densha]

The Sassho Line between Urausu and Shin-Totsukawa only sees one train per day, so I reckon that line is the winner by far.