Like shoes, it's wise to only buy a bike that fits properly. This means not only that the geometry of the frame suits your body, but also that gearing matches your pedaling power. Additionally, you want a bike designed to perform optimally in the manner of riding most suited to your needs -- be that rough or smooth terrain, for recreation, racing or commuting.
There are many, many types and style of bike -- and within even those broad categories, further specialization. Here are but a few of the more obvious types of bike available. Whichever you choose, it will still rate as one of the most efficient forms of transport ever designed.
Whether we'll ever get them as efficient as this imaginative altered rendering suggests, is anyone guess.
Photo: Freaking News' Unicycle Racer (PhotoShopped Image)