If you want to develop go with an iPhone or Android. The iPhone development environment is more mature and Android doesn't charge you $99 a year to install your own apps on your own phone.
If you just want to be a consumer then I'd say it is really up to you.
Android is perhaps the least well designed of the three with it burning through battery like it was unlimited, having an inconsistent UI, and generally being poorly thought out. But in exchange it is entirely customisable, a lot of the mistakes Google has made can be corrected, a lot of the missing piece of the puzzle are replaceable, and you can make the phone how you want it.
The iPhone has by far the best music experience (dedicated sound decoder), has a lot of content (apps, videos, and music), and a very consistent UI. It certainly grants you much less freedom than Android but in exchange there is a lot less that can go wrong. The thing with iPhone for either good or bad is that it is consistent and you know what you will get with it. No surprises.
WP7 is like a more exciting younger brother of the iPhone. It has a dramatically different UI and a very consistent look almost enforced on apps. It has good battery life compared to Android and things "just work" in a very iPhone way. Downsides are the lack of content (apps, video, etc). The fact that tons of stuff requires an internet connection (goes through Bing). The fact that tons of stuff goes through Live Accounts (X-Box Live, HotMail, etc). And the fact that Microsoft has released a tiered app development framework only allowing trusted partners access to certain features, dramatically limiting the competition within the market and almost choking its own platform to death.
I've spent the last few months making the same decision you have (iPhone Vs. Android Vs. WP7) and as much as I like the WP7 UI and the dedicated camera button, I think the ecosystem of WP7 is by far the worst (even considering iPhone is OS X toolchain only).
From what I've read they have given Nokia "special" access to the system to develop apps, just that entire concept that different manufacturers get different app access to their own hardware scares me. The fact that the emulator is useless scares me. The fact that it is $99 to run your own apps scares me. The fact that it was and is impossible to develop a IM-like application without having Microsoft hardcode it into the OS its self scares me. WP7 might never take off because Microsoft created an anti-developer platform. They hate developers. I'm skipping.
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