No-one seems to have mentioned the Shroppie yet - top of my list, atmospheric and love the less used part from Nantwich to Chester
Second for me - South Oxford, love the summit, bend after bend going nowhere in particular
Third Staffs and Worcs especially south of Autherley to Kiddy
Fourth - the vastly underrated Middlewich branch
Fifth - Grand Union, which has some sort of business-like aspect to it which I like. Nicest south of Braunston?
Finally an honourable mention for the BCN, done the challenge a couple of times and the feel of seeming remoteness in an urban area is something I like. Feels like you're really under the skin of the place.....