It's all quite subjective really depending on your point of view. But I'd go with the above comments in that Wales is great, and the tarmac is in good nick, following a load of EU grants some years back. Lots of hills though. I love cornwall, but it's also very hilly. Roads surfaces are a bit rubbish, and there's alot of tourists with caravans that get in the way, but plenty to see and do wherever you are.
Parts of Kent, Sussex and Surrey are stunning. Less hilly that Cornwall or Wales. Road conditions are generally good, but busier than other less densley populated areas of the country.
All depends how far you want to travel for it really, and what else you want from your trip in terms of places to stay, and things to do etc. A good place to start might be the Sustrans maps and website. it'll give you a general idea of where the more scenic cycle routes are, and give you some information on their condition etc.