Out of the box, WinMo 7 wins. It is the smoothest of the operating systems and has widgets that work well and is quite simple and easy to use.
Otherwise, Android has to be the best all around OS
iOS sucks outside of it's smoothness. No widgets, no customization, locked down in pretty much every respect and very in-elegant in overall design, not to mention you are FORCED to use iTunes, one of the worst pieces of bloatware out there, to synch your device and have a retarded file-management system.
Fwiw, widgets/live apps > iOS locked down b.s.