I'm not asking for the one you use now. I'm asking for the one you loved the most, and the one that you wish you could be using today.
I'm a huge Ubuntu fan, but I currently use the Win8 that I got for christmas. I highly recommend both, but if you're building something with a graphics card, do Windows.
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I haven't really tried out Windows 8 yet, i think I'm going to like it though. Right now I use Windows 7.