Windows XP was amazing, but it's old and unsupported now - if you have any sort of modern computer, you want a newer operating system.
Windows 7 feels much like XP, looks better, and takes more system resources... but it can use more system resources if they're available to them; out of the three, it's by FAR my pick.
Windows 8 is a lighter operating system than windows 7, so is better for underpowered computers. That being said, it's got a wonky UI that's designed for touch screens: it doesn't "feel" like windows and takes about a weeks' worth of learning to get used to it.
That being said, there's also the issue of what version of windows to get. I'd always recommend the Home Premium version of modern windows - there are no benefits to the other versions that would be used by an average user; they're aimed at business that have data that needs to be secured and backed up frequently.
As for 32-bit vs. 64-bit windows, there's no question. 32-bit is only good if you have ANCIENT 16-bit programs, which wouldn't run on a 64-bit OS. However, you want a 64-bit OS (unless you have 2GB or less of RAM), because it can use more than 4GB of ram - a 32 bit OS can only use 3.6GB total, including the VRAM on your video card.
Final "best" option in most cases? 64-bit Windows 7 Home Premium.