My favourite is KDE. Why? I'm not sure
I'm not sure I have a favourite distribution. But, 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.