Honestly tough to say. Nile pollution is due mostly to the runoff of agricultural chemicals and heavy metals like cadmium and nickel. Pollution in the Ganges is almost entirely due to human and animal sewage and the disposal of human corpses.

Kind of a column A, column B kinda deal. Do you want skin diseases and cancers from volatile chemicals, or bacterial infections from human shit?

You mean the Ganges doesn't also have chemical and heavy metal pollution?

Oh, I'm sure it does, it's just absolutely dwarfed by the amount of human and animal effluent dumped into the river on a daily basis.

I don't know the answer, but this isn't specific enough. 'Clean' isn't measurable. Suspended solids is. Nitrate/nitrite is. Fecal coliform is. Etc. If you know what kind of clean you are talking about, there will be some numbers available online.

Also, these are huge rivers they will be very different at different points. Finding some average is sort of meaningless.

Ive heard it said about Ganges: dilution is the solution to pollution

Oh sure, but only when the amount of water overwhelmingly exceeds pollution. Hasn't been the case in nearly 100 years.