T-Mobile= best customer care, best deals on phones (all phones for free tomorrow and valentines day). I was with them for 6 years and never dropped a call
AT&T= iPhone and thats about it. The only reason I am currently with AT&T
Verizon= Arguably best voice quality but most expensive
Sprint= More expensive than tmobile and att and doesnt have many great phones
T-mobile is the best choice especially for that awesome deal with every phone including the expensive smartphones being offered for free with a 2yr contract. If you only care about the voice quality and dont mind spending the money,verizon is for you. If you want the iphone then wait until June for the new one. Just don't choose sprint. You can also check out phonedog.com for more info about each one and phones in general.
Where I live right now, AT&T and Verizon have comparable coverage. I dumped Verizon for AT&T and the iPhone over 2 years ago because I wanted the phone. Both Sprint and T-mobile have serious coverage issues in my area. I've had both and dumped both because of coverage issues.
The first thing that should matter is coverage. The cheapest plan and the coolest phone means nothing if you can't get a call out.