That you have progressed on and also in your personal opinion who you think the hardest boss is in the games entire existence is.
Hardest boss i've progressed on would probably be Spine from Dragon Soul and in my personal opinion not based on any sorta statistic the hardest in the entire games existence i'd have to say yogg-saron no lights just because i'm a WotLK pleb and don't have any connections/storys with others.
Just my opinion, whats yours?