We can all agree on Arthas and that's for sure, but i wanted to know if there are bosses that captured your attention, like others pointed out, it could be you loved garrosh lore, KJ encounter, deathwing because he is a dragon, whatever reason you have.
i personally enjoyed Deathwing, during cataclysm a lot, i didn't knew him when he was revealed, but at the end of cataclysm i really liked him, maybe the encounte rwas easy, but the envrioment and the wasy he impacted the world was really felt to me