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.