1) Windows 6.2
: many improvements, including: the Start Screen, Live Tiles (at-a-glance info like weather and news), built in email (killer feature for me) and calendar, notifications, better performance and less memory usage compared to Win7, IE10, built in antivirus, better task manager, improved file manager, App Store, faster startup times, free unlimited music streaming with Xbox Music, sytem-wide spell checking (it even works for Desktop apps, there's a COM-based API for that), & more.
2) Windows 6.1
: the new taskbar (killer feature), Aero Snap & Peek, jump lists, fine-tuned Aero, Direct3D 11, Thumbnail Toolbars, multiple minor improvements.
3) Windows 6.0:
beautiful Aero theme, much better security compared to XP (UAC), newer hardware support, live taskbar previews, Gadgets were cool at the time, new Start Menu with search, .NET included, new Windows Explorer, SuperFetch, Direct3D 10.
4) Windows 5.1:
first consumer oriented version of NT-based Windows, much more stable than Windows Me (not much changed from Win2000), gorgeous Luna theme & variations (I still love them), new Start Menu, very low memory usage, ClearType, built in WiFi support (SP2+), many other performance improvements.
Every single version of Windows has a special place in my heart, but as Bill says, "higher is better".