There are no traditional 5/5 caches on Dartmoor. There is one 5/5 Puzzle cache http://coord.info/GCQKFR (Code Breaker V: CRYPTOLOGY) which is a bonus cache and seems to deserve it’s 5 difficulty rating as the caches needed are all fairly tough. Not having found it I can’t possibly comment on the terrain rating!
There is also one 5/5 Multi http://coord.info/GC2GE1N (Tavy Treasure Trail). This is an excellent cache, which I still rate as my No 1 cache of over 3,000 found and certainly deserves it 5 difficulty rating. It is one of those multis that you have no idea how long it is until you finally finish it and it has a really good final stage. However, I find it difficult to justify its 5 terrain rating as the only special equipment I needed to complete it was a pair of wellies to keep my feet dry.
And herein lies the problem for terrain 5 caches on Dartmoor. A terrain 5 cache is defined as “Requires specialized equipment and knowledge or experience, (boat, 4WD, rock climbing, SCUBA, etc) or is otherwise extremely difficult.” There is not a single river on Dartmoor that requires a boat, certainly there is rock climbing and a cache high in a quarry would certainly qualify, I doubt is diving is allowed in any of the reservoirs, but again flooded quarries would qualify. Of course, caving and potholing would also qualify and maybe there are possibilities there, but generally not a huge scope for terrain 5 caches on Dartmoor.