Skyward Sword, definitely. I love the impressionistic style, and the colors and graphics are just so unbelievably amazing, not to mention very, very good. It really fits the game well, and it's like looking into a painting. It's simply amazing.
I adore the art style of Twilight Princess. The perfect blend of semi-realistic/slightly shaded graphics makes it just wonderful. The scenery was amazing, as was character design. Clothing, hair, everything. The lighting was amazing too and made sure a constant feel was present throughout the entire story. I quite enjoyed the art style, and to me it will be what a true Zelda game looks like. Breathtaking, mood- inducing and all round fabulous.
Twilight Princess' art style was for sure my favorite style, by a long shot. Not only did it create a darker feel for a Zelda game (which is good), it simply made everything look beautiful.
Overall, I love Skyward Sword's beautiful artstyle is the best and has really grown on me with its gorgeous, colourful visuals and stunning cutscenes. The draw distance is my only grip about it but that's mainly because of the wii's graphics capabilities presumably.
Wind Waker's and Twilight Princess' Artstyles are both excellent as well and they do cross about the same for me. In a sense I find Twilight Princess to have the best graphics on the wii although it is not my favourite artstyle. Its still excellent and Wind Waker's graphics are almost timeless.
I loved the lighting in this game more than anything, the melancholic tones of the twilight, the ethereal glow of Link's sword as he rode under the moonlight at night, the rays of light that shone down onto the mountains in the distance and the reflections of the glowing lava in the Goron Mines. Everything about TP just wrote out the word beauty to me in large neon letters. The kind of subtle lighting and the textures of the land and and objects around Link are often lost in cel-shaded installments which really is a shame. Even though Wind Waker still has the epic feel to it, the lighting is no more than conical shapes with no fading and blurring or dissipation into the world around it and while it looks pretty it doesn't look nearly as impressive as walking under the heavenly light of TP's Hyrule field.
I hope WiiU goes the TP route and i hope nintendo realise that with HD they can make things look beautiful and not just impressive.