AD has some pretty tropical-ish and jungle sights, but low variety in it. DC has very bland color palettes but lots of long open space and big cliffs. EP ranges from volcanic mountains to swamps to tundra, but lots of the sights there are... busy, random little stuff getting in the way.
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Each faction is unique in its level of beauty. The Aldmeri Dominion has a somewhat fairy tale feel to it. In Auridon, I wouldn't be surprised to see a unicorn walk toward me out of the trees. In Valenwood, I turn around and expect to see a centaur. The Daggerfall Covenant has that King Arthur and the Knights of the Round Table feel about it. Glenumbra and the Alik'r Desert are both gorgeous. The Ebonheart Pact has an other worldly Sci-Fi aura to it and has its own beauty as well. I'm even learning to love Shadowfen and I really don't like swampy stuff at all.
I listed those in alphabetical order on purpose to show no favoritism to any faction/alliance.
It's all a matter of likings. You may like a green landscape, a park full of waterfalls, a blue ocean, big castles, an African wild life scenary, a snowy forest, a huge lava volcano, etc.
Beauty doth indeed lie in the eye of the beholder!
So for me, it definitely is the pact lands that make the race.
I mean, dominion is woods, woods and more woods, oh, and a little grasspands thrown in for varietee, while covenant are all too classic "high fantasy european landscape" and "1001 nights desert landscape"... sure, it is a classic, but also a tad on the borijng side if you ask me (though well done, I have to admit). Whereas for the pact you get a nice wide seletion of volcanis lands, tentacle plant plains, swamp, snowy thundra and rugged taiga... the only thing better for me would be Vvardenfell, and well... that's coming soon, right?
Really they're all close to me and depends on what your personal aesthetical preference is but for me it's EP for sheer variety. Volcanic ashland, Deshaan with it's lush green and mushrooms, the Rift for a more serene and tranquil beauty, the harsh colds of Eastmarch, and the treacherous swamps of Shadowfen.
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Outside Auridon (probably the only good looking zone in AD), it's just 50 shades of jungle, followed by a bland plain in Reapers Mark.
DC is a blend of everything and nothing at the same time. Glenumbra goes from forest to swamp to cemetery, not really giving you time to enjoy any real scenery, Stormhold is alright, Rivenspire is gloomy and imposing (which I personally like but isn't visually beautiful) Alik'r is a desert and Bangkorai is a mix of Stormhold and Alik'r.
EP has a Volcanic Ashland, Deshaan is good looking, Shadowfen is a swamp, Snowy Tundra in Eastmarch and the Rift is more plains, with small bits of variety thrown in.
Each has it's own charm I suppose but beauty is subjective and no area in the game fits my definition of it.