Well, it all depends on what you want to do with it--for raw power, the Nokia N900 is going to be hard to beat. Heck, you can even just run Debian on it if you feel so inclined!
Frankly (and this is after using an iPhone for nearly two years), I can't fathom buying a smartphone without a physical QWERTY keyboard. Also, for me, I really want something that can be used comfortably as a phone as well as a typing/emailing device--so the Nokia E90 Communicator form factor seems like the obvious winner to me, with a full QWERTY board, a "cover phone" that feels almost exactly like a normal phone (but with the power of a smartphone), and a super high-res display (800 pixels wide makes the iPhone feel like an antique to me!). Also, NOT having touch elements/an onscreen keyboard gives me a much larger working area.
Just my 2c.