The Lukuga -- #40 on the list.

I suspect there is an error in the data. The project lists the Lukuga River as being 1904 km long. But Wikipedia[1] only has 320 km, which would give it a much more ordinary sinuosity of 1.27.

For a river that might actually have a very high sinuosity, take a look at the Fraser River -- #20 on the list, with sinuosity listed as 5.12.

[1] https://en.wikipedia.org/wiki/Lukuga_River

And also numerous rivers with sinuosity < 1, i.e. the river is shorter than the straight line distance between the source and mouth.