For me I have always had the luck of not owning any obviously known problem os (Vista, ME, etc.), however, I have had much trouble with Win7 having the whole os practically crash on me, which makes this my least favorite os. I have tried to help someone install Win7 with Vista installed on their computer, and Vista gave me a ton of issues, so it comes in second. Also, I hate the gui of Win3.1 which makes this my 3rd least favorite os.