Lots of games have awesome skies, and one of the earliest ones I found mesmerizing was Unreal's.

As for backdrops, there's no question. I can stare endlessly at the one from Quake4, and the game used to tease you with levels where you could only see small portions of it. Later on you fight on this massive industrial skyscraper, from where you can make out most of the backdrop for the first time.

A bit of a cheat since it uses pre-rendered backgrounds, but I loved every single location of Bravely Default.

That is lovely. Reminds me a bit of queen Amidala's palace in episode 1.

I read ''bankruptcy''.

Doom 1 sky. Don't know why. Those mountain tops and the creepy green mist. Even though it doesn't at all match what Mars/Phobos/Deimos would look like, it's somehow very fitting and kinda hypnotic.

The night sky in Death Knights of Krynn. I'm so old I used to consider it lifelike. Here it is, obscured by trees but otherwise in all its EGA glory.

If we're going that far back then definitely The Cycles by Accolade. Seemed idyllic somehow, at least until I crashed :P

Either Freelancer or Darkstar One. Can't remember which, but one of them uses images produced by the Hubble telescope for the space backgrounds.

I'm partial to this sky from Far Cry.

This one from Shadow Warrior (2013) is nice too. It fits well with the atmosphere.

Back in the day i loved the skybox of Morrowind at night. Having previously played only older 3d rpgs, the sky of Morrowind looked absolutely amazing to me.

Wtf ? Did Mars went trough terraforming in the Doom's world ? I can't remember.

One of my favorites; in Majora's Mask, there is one dungeon that can be turned upside down. Once you've done that, you can look down and see the giant evil moon below.

One of my favorite games ever has some of my favorite backdrops and skies: Xenoblade, on the Wii.

Following are from Eryth Sea:

http://vignette2.wikia.nocookie.net/xenoblade/images/6/68/Tumblr_m9bpxxBLBW1rdf7a9o1_1280.png/revision/latest?cb=20120909030911

http://vignette1.wikia.nocookie.net/xenoblade/images/7/72/SX4P01-41.png/revision/latest?cb=20131102162427

http://vignette3.wikia.nocookie.net/xenoblade/images/9/96/Syrath_lighthouse-4.jpg/revision/latest?cb=20120504140138

STALKER games in general, also:

http://vignette3.wikia.nocookie.net/stalker/images/8/82/School.jpg/revision/latest?cb=20110806165800

Medieval II: Total War:

http://vignette1.wikia.nocookie.net/totalwar/images/e/e5/Medieval2tw016.jpg/revision/latest?cb=20090910222257

For now, that's probably good.