This list is not intended to be a source of argue.
I will list them not in the order of my preference, but in the order of its popularity.
Remember that they are my top 5 favorites, and maybe not yours so please do not get mad if you do not find your favorite listed here, anyway I would like to hear about your favorite one and why it is your favorite. Ubuntu
One of the fast growing Linux in the world, it appeared 3 years ago, and now it is maybe the most popular distribution. Feisty Fawn its last release is really easy to install and configure, for those who really want it easy, there is automatix which is a script that helps you add all the software and codecs to enable your Ubuntu to work as a great Desktop alternative to MS Windows.
Ubuntu is based on the unstable release of Debian (Sid), so for those of you who like Debian you will find really easy to manage and configure Ubuntu.
You can choose from two versions, the LTS (Long Term Support) and the normal release, the first has support for 3 years on Desktops applications and 5 years for server applications.
It is really easy to install Beryl on Feisty
There is also a version dedicated to students called Edubuntu
You can order free CDs from Shipit
This Distribution came to the world in 2003 (Thanks for the correction Mace), it comes from the well know RedHat Linux, its last stable release, Fedora Core 6 was one the first having 3D desktop support out of the box, we will have FC7 in few more days, the news for that release are the merge of Core and Extras repositories, it will have a Live CD, and of course support for 3D effects.
Fedora release cycle is fast, each 6 month we have a new release, and it is a great Distro to choose to be up to date with all the new software available.
Its installation applications (anaconda) really makes it easy the installation process, but lost of people complain to YUM which is its software management tool.
Debian is one of the oldest Linux Distributions one of its major premises is should be 100% non commercial, it has one of the biggest communities, if not the biggest. Debian is also the base for some of the biggest distributions like:
and a big etc, some of them are not directly based on Debian like Ulteo and Mepis which are based on Ubuntu, and DSL based on Knoppix.
The last Debian stable release is Etch, you can see here its installation screenshots
Gael Duval is the creator of Mandriva which former name was Mandrake, it is also one the first Distribution launched in 1998, when it merged with Conectiva it switched the name to Mandriva, It is primary a Desktop distribution based on its beginning on RedHat, but now it is not yet based on it.
It also use RPM packages and urpmi is its package management tool, it is really easy to use.
This Distribution can be configured almost completely using graphical interface, which could be great for the newcomers to Linux.
Mandriva is now offering a complete Desktop application that can run from a 4 GBytes USB memory. That is also possible with Knoppix or Debian.
Your can see here How to install Beryl on Mandriva
CentOS is the RedHat Enterprise Edition clone, which is free, and comes with no support from RedHat but wich support from the community.
I like CentOS for its stability it is great for Server applications but I have also used it in my IBM T30 Laptop, and as you can see on this post CentOS is a great Desktop application also.
By today these are my Top 5 Linux Operating system Distributions, I will wait for Ulteo to see if it enters into my Top 5 Favorites.
Now you can see a table with the software version each of these distributions latest stable releases comes with. (for Fedora I have consider FC7 as it is really near to be released)