I would say for the origional 4 Enviornments:
Insula Primalis is at a 1. Most of the cards affect least hp targets, which will most likely be the villain's henchmen, and has a global damage buff. Some annoyances do arise, but not too bad. The Starter of Environments.
Megalopolis is at a 3. Some helpful stuff, like a damage buff, but the cards that can come off of that deck can be challenging (can't play hero cards?!?!?), but it isn't too bad. On the 3 scale, it's a little lower on there, just for the occasional filibuster factor. Might even be a two, but I don't think so
Ruins of Atlantis is also at a 2. There are more helpful things out there, but Room Leak and Pillars of Hercules can swing the game in the villians favor very easily. However, the Kracken is a good friend of the heros not named unity, so it's a give and take. Solid two.
Wagner Mars base is a 3. Not a whole lot of helpful stuff, but when stuff happens, it really happens. Expect to skip turns to prevent the base from blowing up and to be able to damage heros again. Luckily, the effects are easily dealt with. Proabably the weakest of the threes, but a three for the same reason Megalopolis is.
Rook City Enviornments:
Rook city is a 4. I haven't even played it yet, I'm so scared to. Very little of the cards help you, and even help the villian. I don't know how Wraith does it...
Pike Industrial Complex is a 3. While some of the vats can help you, like the Irridiated Cyclohexane Vat, some are just no good, like the supercolled trisolvent vat, and the biomemic plasma vat, and when they explode, man! that can be the end for heros. Rats can also be annoying. Definatly raised to a 4 if you are Unity or regular Absolute Zero.
Infernal Relics Enviornments:
Realm of Discord is a solid 2. Some of the stuff help you, but some of the stuff hurts you. It's a little better that the effects dissapear when another distortion comes in, so it's deal-with able, especially if you're Nightmist and can take that damage nullification. Also gets a bonus for perhaps the best flavor text in the set (Imbued Vitality)
Tomb of Anubis is also a 2, or maybe even a 1. My favorite enviornment, a lot of the cards hit everybody and play more cards of the enviroment deck. swarm villians will most likely be foiled by this enviroment for sure. However, since the enviorment gets very card-heavly, any villian that can take advatage of it (The Ennead, Omnitron, Spite, ect.) can ruin your day. An Easy environment, but don't get too comfortable.
Hope that helps, and I wonder if anybody has any other opinions about the enviroments.