Predictably I guess, but the Hampshire Avon. 5 minutes from home and through it's length it offers so many different types of fishing and potential specimens of every species - except perhaps rudd, tench and zeds.
Having said that I would not enjoy my fishing so much if I was indeed limited to one venue.
For me, it would have to be Partridge Lakes near Warrington. Great bailiffs, no stupid rules, extremely weel kept, great fishing and or some reason it doesn't attract many idiots...
But as mentioned already - variety is the spice of life!
A river with a variety of swims, the Hants Avon or Dorset Stour for example - but that would be cheating really.
If it was a lake, then a really enormous one - with bays in inlets etc -which could take a few years just to get to know it. Id be wanting a boat for that though.
Local syndicate lake. Only a couple of miles from home, a lovely place to be and, while the fishing isn't spectacular just yet, it's a challenge and the potential is huge.