I have already mentioned that I am a big fan of SUSE (openSuse and SLED) – re post: SLES12 based Nethserver implementation?
I have had experienced of most distributions (and probably all main-stream distros - eg Suse, Debian, Slackware, Arch, Red-Hat, Gentoo etc.) but prefer RPM based installs.
As far as desktop GUIs, I like KDE / QT a lot (have been using KDE since version 3) but also found that a lot of my clients / end-users prefer to use Mate (I really dislike Gnome 3).
I also like the Enlightment and XFCE projects (also I should mention the terminal / console and ncurses based projects such as Midnight Commander and Yast.)
I am still a user of WinXP as a OS for games and entertainment software (such as XBMC)