Ancalagon
Dusty Dragon
I think this is a good illustration of how super high ability scores can break the balance of the 5e.
Imagine a Barbarian with the ability array listed up top. 20 STR. 19 DEX. 20 CON. That character would have an Unarmored AC of 19, the same as a Fighter in plate with Defensive Fighting Style. The Barbarian doesn't have to worry about disadvantage on Stealth checks, doesn't have to invest a class resource into the extra point of AC, and gets Fighter level defenses with Barbarian-level HP.
And all the ASI can go into feats