If you're near Snowdonia, I can vouch for the Bala to Trawsfynnedd road. Once out of Bala, it twists around a reservoir (which incidentally flooded a village - very sensitive) and then becomes a succession of fast sweeping bends on a brilliantly smooth surface. The road is around 20 miles long.
The A470 is the main arterial road running north-south through the middle of Wales and the area around Snowdonia is fantastic. Watch out for the rozzers though. Happy hunting!
