Monks, DK, and Warrior.
I’ve had multiple tanks in my 14-15’s over 40k. They’re good player also, and well geared. Honestly, if you aren’t breaking 40k average as tank in mythic+, you’re probably playing poorly. Undergeared tanks probably in the 30k range, but geared tanks definitely 40 or higher. Anything less and you probably don’t really understand the game as well as you think.