Want to learn Linux System Administration

I know little to nothing regarding the Linux operating system but would like to. I'd like to legally download a free version of Linux that I could install and work with. I'm not looking for a desktop OS but rather a server OS because I want to learn Linux administration. Anyhow, of all the distributions out there, which would be the best to learn? Bear in mind I didn't say easiest, but rather the best. Thanks.