Android or Windows or iOS??
What would prefer? And why?
Though I've just started to learn Windows Phone App designing using Windows Phone SDK, I'd prefer Android.
Reasons :
1. They are more popular in my country.
2. Files can easily be transferred from phone to phone or phone to computer or vice versa.
iPhone allows only iPhone to iPhone file transfer through bluetooth, I dunno about phone to computer. And windows phones allow only phone to computer and vice versa only through Zune. Both of which I really dislike.
3. User friendly.
4. Wonderful apps are available in free.

Whats your opinion?
Also, do post your vote.

Android, simply because it uses the Linux Kernel.
Which essentially means the entire Android system can be highly modified to suit an individual's needs, much like it's bigger brother on the home front that we all know as varying Linux Distributions.

If you want to design, then I recommend Android. It uses Linux Kernel. That explains it all.
Android is the most customizable of them all.
iOS is a pain coming to customization.
And Windows, well its not that good on mobile devices (from my point of view).

iOS for me, the UI is much better than android and windows in my opinion. To me it boils down to the interaction, I had a windows phone and an android before I had an iPhone, never went back after that. IMO android just seems buggy all the time, while iOS has always been smooth. A jailbroken iPhone is king in my opinion.

Android is like MyBB, customizable to the core.
iOS is like vBulletin and IPB, shiny and flashy.
No comments for Windows :p

Same answer as all above, Android because of it's Linux Kernel...

I have an Android while I prefer the iOS.
I just find the iOS more stable, Android is getting very 'laggy' and keeps bugging out while my hardware of the phone is fine.

Anyhow these are just personal preferences.

Android. There's literally nothing an iPhone has over Android (in my opinion), including Siri. With a 4.1+ device, you have Google Now, which is much faster, there is more selection of devices for Android, bigger screens, more customization, you can root, when rooted you can overclock with custom kernels, flash custom ROMs like CM10, AOKP, etc.. Windows Phone has minimal selection of apps, pretty boring launcher, and the iPhone is just fool-proof. "Oh hey look, I press this icon and it takes me to an app I downloaded" - iPhone user "Oh hey look, I can easily toggle settings from my notification bar, press this widget to turn on my flashlight, use NFC tags to automate tasks, send files to my friends via NFC, and root my phone!" - Android user

(NOTE: these are my personal preferences but really, it's only logical that Android is better.)