iPhone, obviously. Putting your effort into another platform right now would just be fool's errand, or at best would be risky. Perhaps the Pre or Android have potential, but the iPhone is way past the "potential" stage. BlackBerry is another strong option, as they are already big and still have great things on the horizon, but the platform itself is limited and I don't know that a BlackBerry user is going to be grabbing as many apps as an iPhone user is.
Here's why I think iPhone, based on your criteria -
- ease of use - the iPhone may be the hardest because it's unlikely that you would already know Objective-C. However, it's an object-oriented language and Cocoa is an MVC framework. After you get used to the syntax, it's not much different than what you probably already know.
- popularity of platform - this is an easy one. The iPhone may not be the most ubiquitous, but it certainly is the most popular for people who actually use their devices to do things aside from make phone calls and send SMSs and are likely to want to be interested in whatever application it is that you are building. Nobody buys apps for the RAZR.
- cost - The cost of iPhone development is owning a recent Mac and paying Apple $99. If you don't already have a Mac, they are very affordable for a software developer. People will make a case for Linux here, but it's a non-issue, if spending $1000 is a problem for you and you are a software developer you probably aren't making the best career choices.
