Alarak (he can actually get even more).
You know that he’s balanced around his “extra +100% spellpower against Heroes” so he’s not that strong against PvE. The idea that he can deal his real dmg only to Heroes and not that he gets eVeN morE dMg against Heroes.
There are Heroes with higher and or more reliable armor values (I dare to believe that if I’d ask you when do you want more armor you’d answer: “when I’m low hp” and not “when I’m totally fine anyway”.
Having perma unstoppable is quite a perk, but in exchange, he cannot be buffed, healed or interacted by allies and allied units, and there are ways to punish that remarkable feat.
Also, every Hero can be listed in that fashion, mentioning all their special perks and uniqueness. That doesn’t prove they’re op just because they’re unique.
This might help your struggles a bit: