I get asked a lot from customers which I believe is the better of the 3 and given that each have pro and con's its not as simple a task as it sounds, it depends on what you're looking for and want from the phone.
If we start at the basics and work from there, all devices can make and receive phone calls and send text messages so satisfy the phone, after using all 3 I can say there is no difference in voice quality.
The area I live in has really bad 3G signal so I always disable this on all my phones as it gives me a better signal and better battery life.
I will start with the apple phone as that was the first real smartphone I owned.
Apple has always done one thing really well in my opinion and that is music players and I have not found another application like iTunes that works as well and is as simple to use. The Apple Airplay system is also very well implemented and allows you to simply stream all your media from your phone or your computer to your TV or your Airplay speakers.
This allows you to use the Apple TV with your IPhone and computer to centralise your media collection but the downside to this is you need a lot of space to store your collection and Apple devices can be very expensive for the actual hardware you get.
The IPhone also has a huge collection of applications which means that if the phone does not do something there is a good chance someone may have written an application to do it.
The Google Android system comes next in my list not as I think it's any less of phone than Apples but due to the fact that Google just make the software and then phone manufacturers put it on there phones which can lead to a mixed bag of hardware but also means the price can be more easy on your budget, you just have to be more mindful of the phone you're buying whereas the Apple stuff is all controlled and you get a smoother ride.
That aside the Android is a good all round phone with tons of applications on the Google Market and tons more on the internet ready to be installed, this is where Google and Apple differ on their application stance. Apple take the walled garden approach where you can only install what Apple let's you and Google let you install whatever you wish.
Google media service is all cloud based which means you can save your space locally but do require a good internet connection for the uploading and downloading of your movies and music.
I am currently using the Nexus 4 phone with the Nexus 7 tablet and am enjoying them greatly. And for the same from Apple would've cost me a great deal more.
The last on the list is the Windows phone. It mirrors the Apple music service with the software called Zune where you can buy music, movies and application and sync them to your phone. Windows also takes a walled garden approach with its applications and only allows you to install what they wish. Also Windows like Google only do the software so the hardware can fit most budgets.
The Windows phone lacks applications so you may find it hard to replace them if you are coming from one of the other phones, application builders are being very slow to build for phones.
The one big thing this phone has going for it is it's social connectivity, which is the best I have ever seen for a phone. Unlike the other devices where you need the application installed and you can only access them separately. The Windows phone connects them all in to your contacts and displays them all in one button and the metro interface really fits in here.
If you're looking for a media rich phone that has the ability to stream easily to like devices and have the money then i would go with an Iphone but if you're looking for a good all rounder with lots of applications then Google is the way to go.
But if you want a good social all rounder and do not care about applications then the Windows phone 7 or 8 is the best social phone.