Well, the environment does fluctuate in difficulty depending on who you have. I would say the reason they aren't rated is simply that the villain is the measure of the difficulty of the game. The environment may or may not have an impact, but the villain is always important.
That said, here's my completely subjective, unsupported, spur-of-the-moment ranking, hardest first.
Rook City
Ruins of Atlantis
Tomb of Anubis
Realm of Discord
Wagner Mars Base
Pike Industrial
Megalopolis
Insula Primalis
That said, here's my completely subjective, unsupported, spur-of-the-moment ranking, hardest first.
Rook City
Ruins of Atlantis
Tomb of Anubis
Realm of Discord
Wagner Mars Base
Pike Industrial
Megalopolis
Insula Primalis