Which is the best bike and why?

Which is the best bike and why?
There is no such thing as a best bike. Different bicycles are made for different purposes. For instance even the best racing bike will not be suitable for rough terrain riding.

You need to decide what sort of riding your are interested in and then select an appropriate bicycle.
To elaborate on what he said, sort of. If you are into road biking, racing..you know, the "skinny tires", then there are many beautiful bikes out there. I'm 15 and ive been racing for about 4 years, and i have been through about 100 bikes. The one i currently am riding, is probably the best bike i have ever laid feet on. It is a canondale, Caad9 1. It is arguably the best bike frame built, it has the lightest crank in the world, legitimately. You should explore your possibilities, buy a bike that fits you, buy a bike that you are comfortable with. But i would highly recommend riding canondale, they are the best bikes on the road.