I would say that there are not any caches on Dartmoor that warrant a 5/5 rating.
I have attempted 4 hard caches this year (not on Dartmoor). Gallows Gate (GC2HVKK) 3/4 rating, Seven quarries (GC3E0R7) 3/4.5 only three finds since being listed on 6th march, Little Everest 4.5 /5 (GC2EQ12) no finds this year. And the Worms turned 1.5/4 (GCPEV9) – one find this year, the first two were a success the last two not. I can’t think of any cache on Dartmoor to match these for terrain rating (unless anyone knows different, i’d love to try any).