You should pick a service provider and plan first, then pick the best phone from those available. Going in phone first - picking based on what's the coolest - is akin to picking the nicest looking car with no regard to whether or not it's going to fall apart after a few months.
Cell phone service varies wildly depending on where you are. For example, the northeast is pretty much Verizon Country, and the pacific northwest is (or so I'm told), largely AT&T and T-Mobile's strongest area. Talk to people who have them, ask how the service is, and see how they like it.
As far as raw numbers go, T-Mobile and Sprint give you the most bang for the buck, but make sure their reception in your area - and anyplace you may want to go - isn't ass.