I've tried a lot of blog designs- some are cute, sophisticated or simple and others are cluttered looking. Right now I'm using the mima template with some simple html changes. But I delight in change, so it won't stay this way forever! Next time I want something more exciting than the white background, I think I'll try one of Aqua Poppy's designs. Check out Chelsea's colorful blog designs!
I don't think I have a favorite, because there are so many I would love to use on my blog. They are all simple, and follow a lot of the trends in blogging right now.