Cross-country skiing. It's a sport that, unfortunately, many people are unaware of. This is probably due to it taking place on snow. I heard once that a very large portion of the population has never even seen snow.
The sport of skiing is comprised of two techniques, classic and skate. I'll attach a link to a video of each of these. This comes to the first reason why I think skiing is such an incredible sport. A good skier must be able to both skate and classic ski. The techniques are so different that it's almost like two different sports. And if that isn't impressive enough there are several sub-techniques in both skate and classic skiing. Finally if you are a competitive skier you have to be good at both sprinting and distance racing, which are two very different beasts.
The second reason skiing is so amazing is the workout you get from it. Since skiing requires the use of both the arms and legs it is a full-body workout. There are other sports that can also be classified as full body, rowing and swimming come to mind first, but I can't think of another sport that you have to also support your body weight. Supporting body weight in a sport strengthens bones and muscles used in everyday life. Skiing uses the entire body and the results are that cross-country skiers are generally the fittest athletes. The highest VO2 max ever recorded was that of the skier Bjorn Daehlie.
The final argument for skiing is that not only do you support your body weight, but it is also very low impact. Because of this it is very hard to get injured skiing, in fact I have had several instances where friends have injured knees and can barely walk, but can ski perfectly fine.
Skiing truly is a remarkable sport and I know for most people who find themselves at this blog this stuff will be pretty basic, but hopefully this will be helpful to people who don't know what skiing is. And if you have never skied before and have the opportunity to do so I would greatly recommend it.