I voted for Gentoo, although I must say I find ArchLinux must more practical sometimes.
I've used redhat (fedora / enterprise). I've also played around with CentOS and for desktop functionality and ease of use.. I'd have to vote for Ubuntu. It's hands down the easiest for non-unix type people to use.
I use RHEL 4 & 5, Fedora, CentOS 5, Mandriva 2008 & 2009, as well as Ubuntu.
Ubuntu and Mandriva at home for fun and the Red Hat/Cent OS variants at the office.
For anyone that praises Ubuntu... you should take a look at Mandriva 2008 spring or if you like KDE 4 then Mandriva 2009.
I hear all the time how great Ubuntu is and I have to say (after using both Ubuntu and Mandriva) that Mandriva not only has a better set of themes on the Desktop but also that it's tools for just about ANYTHING are not only as good but better in many cases.
The whole Mandrake Control Center is really mature and full featured and the urpmi package management tool is amazingly easy.
Please do not take this as me knocking Ubuntu, I appreciate Ubuntu and it is undoubtedly the most popular distro right now. I just think that many people overlook Mandriva due to past problems and in doing so they lose a goldmine.
Just my $0.02
A lot of people, including myself, are not interested in Linux GUIs or the Linux Desktop, since we use Linux as a server and use the command line.
Hence, a great desktop is not a very strong argument for many folks.
This is why we have a rule not to bash other operating systems. One person might think a GUI is the most important thing. Another person might think that a large user base in the most important. Another person might have yet another requirements, and so on.
So, please post your favorite, but do not put down other distributions. Thank you.
Neo
agreed with boss..
I like the Debian-based distributions the best. Red Hat seems to be catering specifically to system administrators, while the Debian family seems to be targeting a more universal crowd. I prefer Debian stable on my servers, and Ubuntu for my workstations.
Ubuntu is the one I settled on.
I went through a long process of installing various distribs, sometimes reinstalling because I screwed them up so bad, other times just for fun to see something new.
More packages I wanted were in deb only, and I got tired of fighting dependencies under redhat flavors. A noob can fight deps for years and never win 
I'm not experienced with Linux, so I wouldn't even consider anything too tricky. I also required OpenOffice and other similar tools, so that ruled out some others.
Ubuntu was a contender, and for a user like me there's much about it to like. But I wanted Linux for my Thinkpad X41 precisely because XP Pro was chewing a huge proportion of the battery life just booting up. To my dismay, altho' Ubuntu obviously boots a lot quicker than XP, total battery life was even worse.
I know, there are things you can do - but you have to want to do them. I wanted the laptop to work effectively with the least interference from me, that's all.
So when I discovered that Mandriva (with one small tweak - turning off Bluetooth at boot) could deliver c.25-30% longer battery life, it was an easy choice.
I prefer Debian and Ubuntu for all of my installations. I've tried various other flavors since 1999, and I always come back to the Debian family.
I have used RH and Fedora yet. I feel Fedora is excellent and can be used for daily use...
At work we are migrating to the Red Hat Enterprise 5 enviornment from Sco Unix. I'm impressed with the stability of the Hat.
mikep9
I am a fan of debian. I use both stable and release version for my purpose. Debian comes a far way beating all the others. and Debian also has the most no. of packages in its repos.
cheers
Merry Christmas.
Depends....
Enterprise level stuff, I like redhat/SuSe. My personal use and stand alone stuff, I like Debain/Ubuntu.
The first Linux I ever used was Slackware, and while I don't use it any more still have a soft spot for it.
Without paying for anything, to learn RHEL I use CentOS.
Ditto for SuSE with OpenSuSE.
The nicest looking (that I've used) has to be Mint.
I use Suse everyday, the greatest distro for me. And then again Slackware is a favorite.
I like Simply Mepis over all as the most direct type of Linux to use. Its portable just like puppy, installs easily on desktops, reads and writes to NTFS files easily and you can add all the multimedia stuff to it once you find the codecs. 
The one problem is that sometimes it doesn't play well with the touchpad mouse on a few laptops.
Mepis is based on Debian and uses Synaptic (apt-get) to do its updates and so on.
My fav. distro has to be Fedora... it was the first Linux I've ever touched and it was also my training ground (later I switched to RHEL).
Nowadays I barely use it, but you know you always have a special feeling about your first <anything>.
Linux Mint is also the most user-friendly distro I've ever used... even though I'm not really a big fan of Debian-based distros.
My 1st was Slackware in late 1993, or was it 1994? (second beta if I recall correctly)... distributed on 3.5" floppies... used the distro for quite a while until RedHat came out. Switched to Fedora at Core3 and am still running it (FC12) along side an old IRIX6.5 machine (O2).
Just setup a new HTPC running Mythbuntu (9.10). Nice distro, but no RAID so you have to install some extras by hand. Still playing around with it.
I have tried a number of different distrubtions and have opted for Fedora Core on my home main system, OpenSolaris on my test home system, Mandriva on my laptop, puppy linux on usb for troubleshooting windows systems & have now discovered PClinuxOs as a live CD/DVD backup system.
I use GNU/Linux