Have to go with Stuart Harvey, the Great Ouse gets it for me 2. It too has upper reaches, but upper reaches with not only record Barbel but record nudging Perch and Chub too. The middle reaches hold a myriad of fish, Pike, Zander into mammoth proportions, big fish of all species, Carp, Chub, Perch, Roach, Bream and one of my Zander swims two years ago produced two double figure Tench during a routine EA survey! In addition to this, there is always the odd surprise in the form of a big Catfish. Im also told that the tidal reaches are the place to go if you want a bag of Eel sections. The river is full of character and truly is the 'heart' of the Fens!