Based on my personal experience if you are having no problems with Vista you are in a minority. I make a decent part time income correcting, (and attempting to correct), Vista problems. Even though I abandoned Windows on my personal PC's long ago in favor of Linux / Ubuntu, I would suggest that, based on my research, if you simply must use Windows then go with Win7. You are still extremely vulnerable to virus's, spyware and malware, but at least the OS seems to be somewhat stable. Like automatic updates? Well, my Ubuntu system very quietly goes and looks for updates for all the software on my system every morning, notifies me if any or found, and gives me the option to install or not install the updates. And, as Alex Mac pointed out, Linux is free.
I love Windows 7, but since that's not up there, I just voted for Mac because they are extremely similar in terms of style. They both seem user friendly (let's you move the position of items opened at the bottom, groups items from the same program together, allows you to pin the items down, and has the show desktop button easily reachable at the bottom right corner). I'm sure there's a technical term for "item", but I'm talking about the area where you can restore programs you minimized.
I would vote for Windows 7. I used to use Windows Vista and personally, I liked it. As long as your system meets the recommended system requirements, it would work pretty well. The desktop computer I'm using right now was from Feb. 2007 and was Vista before I upgraded to Windows 7 around Christmas 2009.
Windows 7 is a great improvement from Vista and even though I find it extremely similar to Vista, it added useful new features and lowered the system requirements. Hardware also improved significantly (at lower prices) between Vista and 7. With the jump from XP to Vista, so many things suddenly changed, particularly the system requirements, that people just weren't ready. One of the primary issues was program compatibility and this was partially caused by the change in version numbers to NT 6.0. With Windows 7, they kept 6.x so for many programs and drivers, compatibility was no longer a problem.
I still find it funny that Windows 7 is actually Windows NT 6.1. What will Windows 8 be?
I also give an honorable mention to Chrome OS. I'm also running a Cr-48 while on-the-go and even though it's just a Chrome browser, that's perfectly enough for me and Google is definitely working hard to make things stable and fast. My favorite feature of Chrome OS is the fast boot up times and the great battery life. However, I personally wouldn't really compare Chrome OS to the others since its just a browser.