Linux. I love the idea behind open source software, and I’ve been using various linuxes for so long that everytime I use windows I get frustrated that it doesn’t work like I expect it to. Unfortunately, not all software works on Linux, so I still use Windows a lot, but I don’t have to like it.
I’m currently using Mint with XFCE4 as the window manager. I love XFCE but I could take or leave Mint.