Rarity's Secret - Luna V2.0
You'll have to avert your eyes if you're too sensitive for erotic pony art. If you're still here, Longinus-II's Rarity's Secret - Luna V2.0 from the Rarity's Secret series is the most beautiful piece I've seen in quite a long while:
Image: Longinius-II @ DA
The image is fairly dark (naturally), so you'll have to view it in a low-contrast environment, e.g. in the evening. Isn't she beautiful, though?