I've used all three and have found them all to have strengths and weaknesses.
iOS is really slick and there is so much choice, in the app store, that you can always find something for what you want to do. But that being said, you're very much restricted by what Apple will let you do, and what hardware they will let you have. To overcome software restrictions then you can jailbreak, but I found it to be very hit and miss as to what would work how I wanted and went back to stock quite quickly.
Windows is also very slick and the home screen is much more configurable than iOS. I really liked the way it worked and how so much integrated into the design of the theme. I loved the seamless integration with my PC and how quick the device felt (Lumia 520) even though it was the most budget device available. However, the major downside of windows was third party support (otherwise known as the whine of "why hasn't Microsoft got xyz app on their ****** phone") and I felt there were some big name apps missing, or severely curtailed (such as kindle not letting you use your own books).
Android is great and gives so much choice over almost everything that you can sometimes get confused over what to go for. The range of hardware is simply amazing and the fact it is open source means that there are lots of different flavours of Android to try (CyanogenMod, Paranoid Android, etc) if you don't get on with what you have combined with a veritable army of people who are ready to help, suggest (and sneer like arrogant richards, of course) if you have trouble with something. The choice of apps, and the wonder that are widgets, means you can always find something that does what you want, and make your phone/tablet look how you want. The downsides are that there is maybe too much choice, so finding the right handset/app can be a struggle, but I've found most Android owners will be open about the cons as well as the pros.
As others have said, the Nexus 5 is fantastic (typing this post, at the speed of light, on mine) as I've finally found thr lovely slick interface I had on iOS and windows with the dynamic customisation of Android, but you should really try out everything, in a shop or from friends, before deciding whether to switch or stick.