Why did I choose the iPhone over the competition? I've HAD the competition. I started out with the competition. They don't come close.
My first smartphone was the HTC Droid Incredible. I got it on the launch day and it was the flagship Android phone of the time. I ended up spending nearly $100 on extended batteries and about doubled the width of my phone just to get through a day without it dying. About a year later, the phone was slowing to a crawl. Finally the iPhone 4 came out and I was in love. I eventually decided to try out the Galaxy Nexus and the Droid Razr and had similar results to what happened with the Incredible... terrible battery life (6 hours bad...)
After owning these phones, I've come to the conclusion you simply can't find a better designed phone. The attention to detail and design on the iPhone and the building materials are unparalleled. The software is snappy, smooth, and responsive. The apps seem to run much better thanks to Apple's QA and the apps are design much better thanks to Xcode. The battery life is much better thanks to software optimization.
I could go on and on about this...