The most difficult raid I was ever involved in required five people to each press a coloured button in an order that would be given to you, but it was randomised each time, and you had a limit on how long you could take to do it. And we had to do it correctly at least five times to succeed. On it's own, this would not seem like a big ask, but sometimes trying to get a bunch of gamers to do something specific, or even Wait Their Turn (absurd), is like trying to get 12 cats into carriers all at once.
The most difficult thing I did in a MMO was in FFXIV, clearing Ultima Weapon Ultimate. It was probably about 100+ hours of the whole group learning its mechanics and the different ways that said mechanics could randomly affect each member of the party and how we were supposed to respond to them, because anyone making any mistake in the whole ~16 minute fight usually spiraled into a party wipe.
One was enough for me. I don't have the dedication to tackle the other four ultimate raids, and I think they're all still collectively considered the most difficult raids in all of MMO history.
---
All this talk about action RPGs just makes me more impatient for Dragon's Dogma 2 news. Still the gold standard of action RPGs with an actual class system.
