Below are some very personal opinions on some of the most popular Linux Distributions. Keep in mind that any of the below Linux distributions can be a stable server and a nice graphical desktop client.
When you are new to Linux in 2015, go for the latest Mint or Fedora. If you only want to practice the Linux command line then install one Debian server and/or one CentOS server (without graphical interface).
Here are some links to help you choose:
Thank you for your reading, if you have any questions, please comment below, the author will be response your concerns.