Although nearly all bikes have some level of beauty, it's well known that some bikes are more beautiful than others. What is less well known, or at least less well documented, is that some bike sizes are better-looking than others. I give you the authoritative guide on mountain bike sizes:
Small: A little too squatty, sort of like the bike was built-up and then smashed. Add 29 wheels, and it starts to get ridiculous. What makes you feel even more ridiculous is when Mrs. Holley easily passes you on one of said small 29er bikes (although this begins to feel more normal/expected after the first few times it happens)
Medium: A thing of beauty. Heavenly angles separate the front and rear triangles. In fact, if a form of deity were to design a bike, it would be a size medium. Add 29er wheels, and it really becomes the perfect bike.
Large: Still a pretty good bike size. The angles aren't quite as heavenly as a size medium, but it can still result in a stunning bike.
X-Large: Somehow, some of the fastest guys I know ride X-Larges. However, it doesn't change the fact that X-Large frames are too angular and awkward to possess the same level of beauty as a Medium or even Large frame.
Those are my thoughts on frame sizes. I'm not afraid to take on these kinds of tough topics that nobody else is talking about (usually because they're not worth talking about). And if it wasn't obvious already, I just happen to ride a size medium frame.