The beauty of gaming is the ability to feel lifted from reality. The stories that make you feel that you really are a part of it. I am real big on how well the story is written, that is what makes games beautiful for me.
Rich storytelling, the environment, awesome gameplay and most of all, love & compassion that the developers work so hard to achieve making a beautiful game. Witcher 3 is a good example. (And before anyone ask about the gameplay being horrible, know this, I like the gameplay controls when I play it on PC with my Xbox Elite controller)
above all else, I'd say the creativity. How people use a color palette to convey something, or how one person's interpretation of source material is different and beautiful to your own. How something that you can't really describe or convey is described and conveyed, like in Hellblade; I mean how do you experience mental illness without being mentally ill?
Immersion is also something beautiful; the best games can make you forget you are playing a game, and that is magical. You're just along for a ride, but at the controls, too.
This kind of goes hand in hand, but is also at war with, immersion, but I think suspending your disbelief is another beautiful thing. It's tough because we want to ground ourselves in reality even in games, but being able to take reality, and say "Here is how it might look if blah blah blah" and experiencing that unrealistic thing and going "Hmm, OK...I Can dig it. This is cool." is pretty awesome.