We finally have another boss that's actually strong enough to one shot us. We haven't had a boss this powerful since the Lich King. I believe the Lich King could still defeat us but he would actually have to try this time where as back then. He defeated us without trying. But I think it's safe to say Argus took the crown as the most powerful boss we ever fought so far. Seeing as he can one shot us and the fact that we need the pantheon's help in the fight.
So the top 5 strongest boss is probably this. Counting all bosses not just final bosses.
1. Argus the Unmaker
2. Arthas the Lich King
3. Avatar of Evil Aggrammar
4. Kil'jaeden the Deceiver/ Archimonde the Defiler
5. Deathwing the Aspect of Death
6. Yoga Saron the God of Death
7. Juiced up Garrosh the Leader of the True Horde
8. Lei Shen the Thunder King
9. Malygos the Aspect of Magic
10. Algalon the Observer
Feel free to make your own top 10 list.