What phone should I get? iPhone, Blackberry, or Android phone?

My 3 year contract will be up soon (Bell, in Canada). I am dying to get a smart phone but I’m not sure which is best.

I am definitely a heavy user, and will be using the internet and downloading lots of different applications. This makes me lean toward an iPhone 4.

What are your thoughts? Does anyone have an opinion about which of these is best? If so, why? What are the pros and cons or each?