Dartmoor is the best option IMHO and south west of Moretonhamstead, the climbs tend to be reasonably sustained but not too steep.
East of Dartmoor, the coastal regions and South Hams are comprised of repeated small (and not so small) steep climbs that can prove challenging if you plan a route avoiding the often v busy main roads. Unfortunately these single track adventures are also not necessarily free from White Van Man (I speak from experience) who uses these vans to avoid the main drags.
H