I think Age of Conan did dungeons really well. Niche MMORPG from smaller "indie" company.

Khitai expansion (sadly the first and also the final one) offered various of dungeons with dynamic normal and hard mode triggers on the bosses. You could perform specific tasks to activate the optional hard mode which by the way gave better rewards for those deserving them after clearing the boss. You could still solo farm the hard mode tokens but it literally takes forever, at least it did when i played. Few tokens per day and item costs 500 tokens. Good push to learn the hard mode I'd say. Dungeons weren't so scripted so you could clear them with knowledge and gameplay experience. Depending on the class of course.

Heck, they even had Vauthry's distant cousin Po-Sha and he died when demon born out of him. Hit's like a truck.
His hardmode you would activate by attacking him and his elite guard all at the same time without luring the guards away from him like you would normally do.

Would go back 10 years of time just for that one game. Slightly more modern than Classic WoW with mature themes and small tight community