First of all, it's mostly about the service, not the phone (mostly). And no service is best everywhere. Talk to friends and ask what they use, and what their experience has been. Ask about dropped calls, no-service areas (dead zones), and about the companies customer service.
Once you've narrowed the service provider down, then do Internet searches for reviews for the best phone they sell (the "same" phone may be different inside when sold under different service providers). For me, the most important criteria are 1.) reception (this will vary by phone under the same service), 2.) battery life, & 3.) voice quality (in both directions). Color screens, camera, etc. are waaaay down the list as important features (again, for me). You can pick and choose what you need most.