We have visited dozen or so escape rooms so far but the one we have booked at Exit19 (Prison Escape) was definitely the most difficult - only afterwards we learn that it was expected to be difficult, as it was developed for some "escape room championships". We didn't do well (I'd better pass over our time and the number of prompts we had to ask for), however I have to admit that this place was very well arranged and is run in a very professional way.
Of course, we were disappointed with our performance, but it would be unfair if I didn't award Exit19 with the highest rate.