I'm not sure I have a favourite distribution. The first disto I used (1999) was Debian. So I'm used to thinking first in terms of "apt-get/dpkg" rather than "yum/zypper/rpm/...".
My favourite distro tends to change every 9-18 months, butat times it's been Debian, SuSE, Ubuntu, Mint and Fedora. The longest as my favorite would be openSuSE
Ubuntu, Debian and Mint seem to have the best variety in default software repositories, and I prefer Debian on my 7 rack servers, though ~half of them (3) are CentOS, because Red Hat coded 'modules/drivers' were better for some prolinear/poweredge servers. I love how Ubuntu/Mint auto-play movies and mp3 music without having to load codecs separately.
If you want a distro to be reliable, I have found openSuSE the best for desktops, and I would recommend it for workstations. My choice for servers is Debian, though as stated some server hardware is easier with CentOS. Fedora is not unlike openSuSE, with more English software choices in default repositories.
I LOVE GRUB. It lets me select a different distro each day, and I can pick according to my mood, or whatever I need to do that day. On most machines I have three distros installed, storing all data on servers.
As I write this, I have OpenSuSE, Fedora, Ubuntu on this machine. Local drive stores only OS and programs.
I use many machines. Some are old Pentium II/III/4 with 512Mb which are best with LXDE (light desktop). Puppy is best on prehistoric P-II/III machines, even coping with original PCI sound-blaster AWE-64 cards that other distros won't use. Sure KDE/GNOME runs on them, but is slow with software loaded. XFCE has a little more eye candy; but on these older machines I'm mainly using text based tty's and virtual-terminals. Cutting/pasting text is the most sophisticated use of GUI I'll use there.
My better workstations are where I do GUI based work, with >3Gb RAM. I love the idea of Cairo-dock, but it takes too much screen real-estate bottom centre. I like Unity shoving it left of screen on widescreens, but get annoyed with the menu in Gnome 3, so prefer classic-SuSE, Mate/Cinnamon/LXDE/Xfce - or better yet sticking to my beloved KDE.
Yes, my favourite desktop environment is KDE.
Why? Sorry, I'm not sure, but believe it's the ability to put widgets on the desktop clock, cartoons, weather). It just suits me. I love multiple virtual desktops and the way KDE does it. I love KDE's menu system, desktop configuration options (decorations/etc) and the Dolphin file manager.
Today I mostly used KDE as I did yesterday. My wallpaper is auto-changed every two minutes from a directory of my favourites. Cartoons flip every 24 minutes, even if usually covered by real work. Its there if I need a few seconds break!
Tomorrow my desktop choice could be something different. I'll select at login prompt after selecting a distro at the GRUB menu. Isn't Linux great!! Choices+!!
This article by Chris Guiver won the prize in the Desktop Environment articles contest which Linux notes from DarkDuck ran together with BuyLinuxCDs.co.uk site.