For me personally:
Android is too messy, customizable, and cluttered. There's a LOT you can do with android, and pretty much every app in the Play Store is built up better than their IOS or WP8 counterparts.
IOS is the exact opposite. It's boring, but does work very smoothly and reliably. IOS also has a lot of apps, of which many are great looking and very stable.
Windows phone is a great in-between. It's fast, fluid, and not cluttered or messy like android is. It's got it's own ecosystem (office, Bing, etc), and it's also a very clean and sleek looking system. The app selection still needs a bit of work, but it's getting there. There's pretty much an app for everything, but some are third party apps, and you sometimes have to sacrifice stability and function for that.
I have all three systems right now. I'm on my HTC One right now, using android 4.1, and I've also got a Lumia 920, running WP8 (not Amber, yet), and an iPad with IOS 7 (beta 6). If I had to choose one right now, I'd stick with my Lumia 920.
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