Debian GNU/Linux because of its emphasis on free software. also, it's an operating system that doesn't make me feel its presence. couple it with a stable desktop environment like xfce and it becomes a good combo. I've installed it on all of my machines. be it server or home devices. it's my universal operating system.

though in office I'm provided an ubuntu machine, with which I'm also content since at the end of day, it's GNU/Linux. it's all that matters to me.