Thinking about this one before I vote. As you say, Warren there is more than one "Best River".
For me, the best River to fish is your own Derbyshire Wye, either Monsal or the Peacock Water but...........
I was once lucky enough to fish a private, restored beat on the Wiltshire Bourne (might be Hampshire, not sure).
To fish in the footsteps of Harry Plunkett-Green and recognise many of the Landmarks in his superb book was a unique experience I shall never forget.
Nearly impossible to catch fish, never seen water clarity like it and the fish all had rear Gunners who could spot you coming up behind them from 30yds. away!! The banks have been left very overgrown, in parts, which made a long cast impossible.
Thinking on as I can't go fishing today