Also agree that Superior 's cover art is awesome. 2nd Tegami Bachi. Yaoi cover art: The Summit. It is pretty detailed, though it looks way too flowery and stuff for me. And Magical Jxr cover art is very... sparkly & cool.
________________ Simply another normal human being...
Oh, this looks fun. I'm going to interpret "cool cover art" to be on those covers that are well-drawn (obviously) and that also have caught my attention (strictly in the positive sense, of course) for some particular reason. Seems fun because, while I see the "well-drawn" part to be objective (except for how strict one's standards are), the extra somethin-somethin that makes it cool is really open to interpretation. What is it in a work of art that you take to be extra special? Well anyway, the only one I can remember offhand is volume 3 of Biomega. (If I can find a better image, I'll link to that, since you'll need to see the details well to see why I chose it.) When I feel like sifting through my stuff, I'll add more.
Edit: -Kakehagi no Hito. I like this idea of trying to capture the aesthetic beauty of something in the simplest way: Since there are so few lines and no color, each stroke must be perfect. So it's like the artist uses some of the ideas from calligraphy and applies them to the drawing of pictures. -Shoukoku no Altair, all covers. I was impressed by the amount of stuff (and the layout, since having so much stuff requires skill in positioning it so that the overall look is attractive and coherent) and level of detail. -Arashi no Hana Murakumo no Uta, volume 1 and Amagoi, volume 1. To hear me complain about ugly art, you wouldn't know it, but actually I'm easily impressed by certain things, like pretty colors. These two focus on colors so heavily that you could say they "explore" color, and that's what I like about them. The first one is about the many shades of red, from the striking classic red to the palest pink. The second is like a little clip of the spectrum, going from watery to pale blues, then to blue-green, and finally to earthy green--I like this not just because of all the shades, but also I've always thought it cool how one color turns into another one. Okay yes, I'm a dork.