I've tried WP, Android and iOS with the Lumia 920, the HTC One (M8) and iPhone 4s/5s/SE
Windows Phone was alright for the most part. Some uneducated people will say it doesn't have apps but the Windows Store is definitely well equiped. You have all the essential apps (sans YouTube iirc
) and a plethora of other apps that work just like they do on iOS and Android. Its design is sleek but a little boring after a while. It's devoid of color and I got over the Live Tiles fast. There was the Swype keyboard which is imo the best keyboard on any phone OS but that appeared on iOS and Android as well. Cortana is alright. Really, unless there are some apps that you definitely need on your phone, WP is a good and safe choice. I see it as the best of both worlds; not nearly as restrictive as iOS but sleeker and more "polished" than Android.
I didn't like my Android experience very much, partly because my phone (the HTC One M8) wasn't all that great. I thought the camera absolutely sucked and a lot of apps would crash unexpectedly. The UI I had on the HTC phone was great looking though. I completely despise Samsung and Sony's designs though. If you're looking for a phone OS that will let you do what you want to do with your phone and tweak every single aspect of it, Android is great for you. But keep in mind that the phone is vulnerable and I've seen instances of people accidentally bricking their device because they either tried to root it or their file system got corrupted.
iOS is, in my opinion, the most elegant phone OS and the safest choice. You have millions of apps, iPhone's an ultra speedy phone (will outspeed your flagship Android phones in practice, not on paper), and has great apps bundled with it like the Apple iLife suite (Pages, Keynote, etc) and GarageBand. The OS was tailor made for the iPhone so you won't have any sort of security breaches or viruses. It's a perfectly safe OS. The UI is full of vibrant colors and transparency and translucency effects, lots of eyecandy animations, menus easy enough to navigate through, some very convenient multitasking and general phone use, etc. You really can't go wrong with iPhone and iOS unless you want to tinker with your phone.
I personally prefer iOS but you can make an educated opinion for yourself by reading reviews and comparisons that highlight all the dfiferences between the 3 main mobile OS's. Don't look at Blackberry as that thing is dead in the water.
I'm not too impressed with the androids I've tried (still though, I wanna try more). iOS >
