I think the best out there is Windows XP. Windows XP doesn’t use as much as ram as the others one, it’s faster and you can change it looks to match your taste.
Windows 7 is good but It has a lot of unnecesary things as well, they just keep the things that are important and not other thing that are useless to most people and causes heavy use of ram and loss in speed.
The vista os is similare to the window 7 but main problem with the vista is the speed is very slow.