Those axes are some of the best melee weapons in the game. Not only does the purple variant have exceptional DPS and crit damage for it's skill requirement, its special move does double damage in addition to the double damage that all VATS melee attacks have. This means that a sneak attack with a maxed out melee skill and perks can one shot a standard deathclaw, with the only the X-2 Antenna beating it out for raw damage.