I was never one for PVP, so I'll keep my opinion on that out of it.

But I did the "Cutting Edge" for every boss and MoP had some of the best bosses, still to this day. And, unlike many raids, it actually had more than one or two per tier. But every expansion has good fights among the trash, I know people who claim Denathrius is one of the best bosses ever.

If we are going to talk about mathematical impossible bosses though, Legion contained Helya. One so far out there that many guilds just didn't bother.

Heck, I've been a feral druid since Vanilla and MoP was the best the entire class has been before or since, and they've been chasing that balance ever since.