Aside from having to have silver or better weapons in order to damage them I thought scamps were one of the best mob types of all time, an IMO shouldn't even have made the list.
That being said, I was going to vote mudcrabs, but then I saw the other post nominating Cliff-racers. And I agree entirely, cliff-racers traveled in packs an would never leave me be... to the point I would cringe in shear annoyance at the mere sound of that chip/chortle sound the always made lol.
Half the time you can't see them, so you're stuck in combat, running around looking for them when they might be under a rock somewhere. --- "When the gates opened, Argonians poured into Oblivion with such fury and might, Dagon's Lieutenants had to close them."
Seriously, how could anyone overlook cliff racers, but another honorable mention is rats and those damn kwama foragers. Survived untouched vs dremoras and golden saints but always took damage from cliff racers, rats, and kwama foragers. Nothing was funnier than falling on your backside while levitating across Morrowind. --- "Grown-ups never understand anything for themselves, and it is tiresome for children to be always and forever explaining things to them."