Either Doomclaw or Spirit Fox.
Doomclaw because that low devour percentage that hits every time is just infuriating. I mind devour less at lower rarities as a way to give people with a smaller collection a chance against overpowered mythics, but it’s a lousy ability to put on a mythic.
Spirit Fox because it literally is uncounterable mana drain. It’s super annoying on yellow GW days as well, because your first turn is almost always meaningless, and then you get to start your second turn with the worst board imaginable and one of your troops half dead. It doesn’t help that the troop is often combined with other (more fair) mana drainers. When Spirit Fox cascades into mana for the other mana drainers, that’s really demoralizing.