Windows 7 vs. Windows Vista vs. Windows XP

[Googolplex]

Windows XP Professional x64 Edition

That's the best Windows ever!
Well you couldn't use DirectX 10 or 11, but all the games are compatible with DirectX 9.
You doesn't have best graphic-quality but the Windows works very well and fast.
It's the once fully 64-bit-Windows. Vista-64 and 7-64 doesn't have full 64-bit-address-space.

In future you couldn't use XP-64 because its too old.
But the once point is the old DirectX-support.