I love Hinohara Meguru's Secret XXX and Therapy Game. Her main characters are just so incredibly pretty, I can stare at them for hours. UraBoku and Silver Diamond also have incredible art. But my all time favorite is Yasashii Ryuu no Koroshikata. I've never seen art as intricate and beautiful as that one in any other manga, yaoi or not.
Not yaoi but yuri. Jaeliu's Beloved has the most breathtaking watercolour scenes I've ever seen. The 'sex' scenes, if you can even call it that, looks so creative and elegant. The expressions on the faces of the characters are full of emotions. I don't think I've seen any yaoi that emits the same feeling as this manhwa.
Oh but I do admire the way Ogeretsu Tanaka and Harada draw their characters. They have the most expressive faces.
Currently I really love the look of this one: http://www.mangago.zone/read-manga/the_wilting_light/
I also love the art from Gusari, Scarlett Beriko, Zaria, Old Xian and Nakamura Asumiko. I would say that the last one might have to grow on you though. Nakamura's style can look pretty strange when you first see it.
What's the yaoi manga that you've read so far that has the most beautiful art?