4A has been overall the most competitive from my prospective. Many times state medalist stay home in 4A, just a matter of numbers and talent, the more wrestlers on a team, the more chances of having more talent at the regional tourney.
All classes have great individuals and any given year a great wrestler in any class could defeat all.
3A regional in the Northwest is ALWAYS very difficult.
6A and 5A have there difficult regionals also