No OS is going to be able to completely replace Windows if you need Windows programs. That's a fact. A sad fact but a fact nonetheless. Wine does a brilliant job, but earlier or later (most likely the former) you will come across a program that glitches or refuses to run altogether, even after hours of tweaking and reading guides.
Having said that, I'm a full-time Linux user, and I'm very happy with it. You'll just have to learn to use software made for that platform. Practically all of the software that I used to use in Windows (Open/LibreOffice, eclipse, NetBeans, Gimp, Inkscape to name a few) run natively under Linux, so I've had very little issues finding the right software, but for you it might be a different story. The only thing I actually use Windows for are games, hence why I'm clinging onto my Vista installation.
And to actually answer your question: Windows 7. It's snappy and mostly rather stable too, and given Microsoft's good track record in supporting antiquated software you should be a-ok until Christ's second coming or so.
What do you actually use your computer for and what programs do you use? Game a lot?