I love this topic! It brings back fond memories!
I myself proposed to my wife at Kamogawa river near Pontocho. I was holding a can of Ebisu black in one hand, and a ring in the other
Kamogawa isn't necessarily breathtakingly beautiful, but the atmosphere of the shallow river, mountains on the horizon, some people enjoying their spare time...it's my favorite spot in Japan. Pontocho will be a bit busy, so it depends on you whether you like that or not. You can go up north (between Sanjo and Demachiyanagi) for a quieter spot.
The options above are also very nice!