kent has a good case (as it faces off to france), but can't help thinking it would have been one of the welsh counties as the english peppered the border lands against attack and also in the counties to consolidate their gains
Herefordshire in the early Middle Ages had more castles than any other county in England. Northumberlands website claims that as a result of the border dispute between England and Scotland, Northumberland now has claim to a greater number of castles than any other English county. There are more than 1400 in England. But I think that Kent does have the most, alot of them are privately owned.