You will need to specify what you mean by the best operating system, because it all depends on your own personal / professional computing needs. You won't need the same operating if your are a graphic designer, if you are a web host or a video games addict ...
In terms of graphic: Mac OS is best but only if you have a 128 bit or higher processor
other wise Windows is easy to use & understand & troubleshoot but still unstable & costly and also you have to buy an antivirus, MS word etc. (another weight to your pocket) .
And if you want to go for free then Linux is good. And in Linux, Ubuntu Linux is more easy to understand & use, troubleshoot.
About your Windows 7, I would rather suggest to downgrade & use Windows Vista but if you can't then in Windows 7, Check for Updates daily and also mail me your problems that you are facing with Windows 7, I will be happy to help you.