We do not officially rank the difficulty of our rooms, but The Bank does have the lowest escape rate.
That being said, any of our rooms are really good for highly experienced players. Our hint system is designed so the players have control over when they ask for help, and all of our rooms are challenging, so groups of all levels will be challenged. Think about themes when scheduling: Do you want to save the world? (The bunker) Do you want to strike it rich? (The Bank) Do you want to see if you would survive being kidnapped? (Zodiac) Do you want to escape prison? (The Prison).
Some other helpful links: