I've been using Linux Mint with KDE on both my desktop and laptop recently. It's improved substantially since I last used it a couple of years ago! Overall, it's quite the pleasant experience. I just wish I could get my Radeon R9 290 and Intel HD Graphics 4600 to both display something on their respective monitors simultaneously.
If I find the time to manually tweak and configure everything, I may install Arch or Gentoo. Mint definitely gets the job done for now, though. It's sleek, beautiful, and quite easy to use!
Fedora 20 on a System76 laptop. It came with Ubuntu preinstalled but I've used Fedora happily for years and never tried Ubuntu. I left it on the laptop while I tested the hardware and when I was happy I went straight to a new Fedora install.