All depends on what carrier. For T-Mobile, I'd say hand down HTC HD2. It runs windows mobile and the screen is very impressive. For Sprint, go with HTC Evo, this is like the HD2 but with android operating system. Also is HDMI compatible, camera on front and back. For Verizon I'd maybe say go with the a Blackberry, for the Droid. I love my blackberry and it's an older 8330. And for AT&T, go for the iPhone, the 3gs is a lot faster then its older 3g. Also can do video now and auto-focus on the camera. And we all should know 2 things by now. Next month Apple should be releasing the new 4g iPhone and also the OS 4.0 operating system will be coming out. This means that the iPhone will now be up to par with android operating systems meaning that it will be able to multi-task apps. That is the biggest downside of the iPhone now and some people don't like the app store but the androids because of the open source.
Right now I work for the largest phone retailer so I get the inside scoop on items, new releases ect. If you have any questions send me a PM and I'll do my best to help you out.