There are many Linux distros and we have unique reasons to like them. To fully master Linux, start with the top of the list - the most user-friendly - and slowly choose the next one, until you reach the bottom of the list - the most challenging - where you can get your hands dirty.
Ubuntu / Linux Mint/ Debian
These distros come from a single family line. Linux Mint is based on Ubuntu, and Ubuntu in turn on Debian, that said, they have unique appeals. Ubuntu presently has the biggest share of the Linux users, thanks mostly to user-friendly installation process, a desktop developed to accommodate beginners, and a rigorous schedule for new releases. Ubuntu is bundled with a wide range of open-source applications to cover the needs of novices. Debian dont come with as many programs, but users can easily download over 25,000 programs from Debian, 3rd party repositories, and web sites like GetDeb.net. Linux Mint shares many similarities with Ubuntu, but aims for a green-themed, clean new look. The distro has a strong focus on better functionality and a large set of drivers in its installation procedure. It uses Mint Tools, a collection of wizards and apps that strive to make managing and configuring Linux as easy as possible.
Fedora
Fedora is Linus Torvaldss distribution of choice. It is a spin-off of Red Hat and came into being in 2003. Although technically a newer distribution, its a solid and seasoned Linux release with a strong user base and corporate backing. It comes in a few "spins," tailored with different functionality and apps, so you can get a Fedora flavor that you need without having to hunt them all down.
openSUSE
While it is not quite as easy to install as other distros, openSUSE is relatively user friendly. It includes ‘Yet another Setup Tool’ (YaST) to make adding applications a breeze. The UI is based on Slab, a glassy Windows Vista-like environment. openSUSE puts its emphasis on ease of use and hardware support, making typically difficult tasks like adding a second monitor less awkward. OpenSUSE joins the SMOLT driver project; where you can choose to participate in a survey to improve support for new hardware.
Arch Linux
It is a lightweight Linux distro which strives to keep everything simple. Its unquestionably not a Linux distribution for beginners, as it installs with no GUI desktop and just gives you a bash command prompt. Arch Linux is a spartan and lean system, but can turn to a highly versatile OS.
Gentoo
Gentoo is even less user-friendly than Arch, as you essentially need to build up your system from scratch in the initial installation. Sure, you can retrieve an entirely pre-packed version and perhaps a live CD, but the common way to install Gentoo is by compiling a unique configuration specific only for your software and hardware needs. There are tons of options and choices during installation, but they are pretty clearly defined. Some users noted that, although they were previously started with other Linux distros, it wasnt until they started installing and using Gentoo that they really know how Linux really works.