Best Art: Chalice of the Void
Everything about this art just works. The crazy evil guy hovering over the cup, the cup itself, the mist billowing out of the top, it's just fantastic. It's one of those pictures that I'd want to frame and put on my wall because it's just that good. It really gives the card that "evil" feeling, which if you've played against this card, you know fits.
Worst Art: Vanguard of Brimaz
This is a good piece of art, but it really clashes with what I think the card was going for. This card was supposed to showcase Brimaz's elite soldiers and it kind of looks more like a soldier-in-training. It also looks like the lion in the background is yelling something at the one in the foreground and it spooked him a little bit. Good art, but it clashes with the spirit of where it was put.
Art I'd Want Painted on the Side of my Van: Godhead of Awe
This came close to being the best art, but I figured it was too metal to be a white/blue card. That being said, it's so metal that I want to get it painted on the side of my van (should I ever get one). If you look at it without the border, you'll see what I mean.
Mark Zug has done a lot of great work for Magic. It's so good that his worst art isn't actually bad, it just clashes with the intention of the card. Definitely check out his website for more great art.