Whether it be a level, or a whole open world, what environments in video games are your all time favourites, and why?

Do you mean a specific level/ place in a game?
Or the Overall levels and location/environment of the game?

Not an easy question, but I'll try to answer it the best I can.

In no particular order:

The garden of Elohim - The Talos Principle

A somewhat made-up name (though I think the author envisioned as such) regarding the world you inhabit in The Talos Principle.

This game brought such a tranquil and alluring world(s) to explore. Based on the idea of the Garden of Eden, including the forbidden fruit (which is actually a building in this case), the Garden of Elohim was one of the most interesting settings I've been in a puzzle game. I'm tempted to say I've already seen it in Serious Sam - considering they're basically the same assets - but the way they were presented here brings another dimension into the table, which is why I precisely love it so much.

Majula - Dark Souls 2

For all the flaws it has over its predecessors, I definitively think Dark Souls 2 nailed it when it comes to the nexus you often visit whenever you have to get some items, level up or just chill around without the worries of being killed. The music, the atmosphere, and the NPCs (while repetitive dialogues) going their way here made it really a safe and nice place, considering the dangers of the overworld and every other corner in Dangleic.

Bowerstone - Fable 2

Such a nice place to start your heroic business! Or getting a haircut! Or working! Or dancing in front of the plaza!

Willamette Parkview Mall - Dead Rising

Because shopping has never been more chaotic and fun than in Willamette's.

I'll think of more later.

Club penguin

I love roasting little kids and getting banned.

This, though it can be the whole game world if you like.

My favorite is in Donkey Kong Country. The part when you are in the jungle to be specific

Gaur Plain - Xenoblade Chronicles

Anor Londo - Dark Souls

Every snow level in existance.

Great Fairy's Fountain (Ocarina of Time/Majora's Mask) - 

Such a nice place to be at... the music is amazing, the enviroment is pretty and the flying fairies messing around are just beautiful to watch. Plus, all the healing the place provides and safety feeling make you relax and want to stay there forever.  <3

Parm Town - Grandia

For some reason, Fenmont from Tales of Xillia really captured it for me.. had an epic feeling from absorbing the environment like never before..