Raspbian is perfect for beginners to the raspberry pi.
- It is easy to set-up and use
- There is lots of support available on-line for it, as most people are using it.
- It is based on Debian which is a widley used OS.
- It is specifically designed to run on the raspberry pi.
However it all depends on what you want to use your Pi for, raspbian is great for general use. But if you were looking to do something more specific, like set up a media centre you might want to use RaspBMC