Android is really good,
Windows Phone 7.5 Mango metro is amazing,
iOS is really good + bearing its shortcomings but if you are app hunger then iOS is for you,
BlackBerry 7 much better experience then previous BB OS's. Its business oriented.
Android always introduces better hardware,but i really dont like the fragmentation of Android. Its everyday a new droid day.
If you want the best android phone then go for Galaxy Nexus.