Ok I would choose Android & heres why:
Android is not AS user friendly as iOS & thats why a lot of people dont like it. Also a lot of people say Android is slow. The speed of Android depends on the phone you get. I have an HTC Sensation & there is never any type of lag. Thats also another thing I like about Android, the fact that there is a wide variety of phones to choose from instead of just, the two iphones (Iphone 3Gs & iPhone 4) or 4 or 5 blackberrys. Like I said before, Android is not the most user friendly, but if you take the time to learn it, like I did, I promise you would LOVE it.
Now dont get me wrong iOs is not bad. I must say that I love it to. But, iOS is not very customizable & also misses a lot of features that android has. One of the major advantages that Android has is that it can run flash player. If you know the slightest thing about computers, you would know that 95% of all websites have or use flash content on them. Also iOS does not have TRUE multitasking. What Apple calls multitasking, is pretty much app suspending.
Now blackberry is pretty far behind iOS in Android. Blackberry OS has less features & wayy less apps than both iOS & android. Personally I think blackberrys are overrated & the only thing thats still keeping them on the market is BBM.
Also last time I checked, Windows Phone 7 sucks! When i tried Windows Phone 7 back in January, there was no copy & paste, no multitasking & many other basic features. I thought, "ohh, im not going to need these features", until I started listening to Slacker radio, since Windows Phone 7 didnt have Pandora. & every time I got a phone call or text message, I would have to exit out of Slacker Radio & do what I had to do. Then start back up Slacker Radio & start all over again. It was a pain in the ass. I heard that they were suppose to release an update to include multitasking sometime around this time though.
Oh btw, I work in a cellphone repair/hacking store. lol.