1) Tips for beginners
First and foremost rule - use the distro that is used by your most familiar to GNU/Linux friend.
If there is none of such friends then your choice is Ubuntu.
Why? Because Ubuntu has become the de facto standard for workstations. All new software is made for Ubuntu first. This distro has a huge community, who wrote a great number of articles and howto`s on virtually any issue that may arise from the use of this system. Moreover - these articles usually do not require in-depth knowledge of the system. This allows in most cases to solve a specific problem by simply copying commands and not delving into the essence of what is happening.
2) For those who've seen GNU/Linux, but it was long long ago in galaxy far far away...
There are several options, but the main are:
Debian - old, stable and with the largest repository. The principal changes are rare, and are carried out so as to enable a smooth transition to new technologies. The basic principle is stability - releases are made not on schedule, but when everything is stable and ready. Debian is so stable that even a test version is often more stable than releases of less conservative distributions.
Ubuntu - a relative of Debian, but less bothering with issues of stability and generally running in front of the locomotive. Sometimes this leads to a dead end. Ubuntu developers also love to write Ubuntu-specific software, which often does not live long. Their feature - the convenience of the user. Convenience at any cost. Sometimes even at the cost of the convenience of users.
RedHat and his descendants. Pay close attention to it if you see yourself in the future as system administrator/architect or something like it. A large proportion of enterprise systems running the Red Hat or his descendants. Drivers for special hardware (RAID-controllers, etc.) primarily prepared for these distributions.
3) I am already GNU/Linux user! What can you, pitiful FAQ, advise me?