Windows 7, provided your system can upgrade to it.
its mostly a matter of preference...
The tech side is: Windows vista is fine and less System intensive, but Windows 7 has less bugs, but is more System intensive...
Windows 7 is better than anything that went before it by a country mile, and that comes from someone who has used every consumer Windows version since 3.11 twenty years ago.
Provided you keep a clean machine ("clean" as in no malware or disk errors) it's impressively stable. I've been running a custom-built Windows 7 PC for 3 years and Windows itself has never crashed once even when some 3rd party apps have.
By contrast, my Vista laptop is frequently giving me hassle.