There is no best line.But from bank fishing a floating line will cover a lot of depths.
If there are no fish showing on the surface, then they must be sub-surface.

so you have to search the depths till you find them.

But then water temprature comes into it.hot water fish down/cool water fish up.
Then cloud cover comes into it no cover fish down/overcast fish up.
So now you see just a few variables.There are so many more .That the thinking angler has to work out for himself/herself .
But thats why I love it.