My honest opinion...for what it's worth
Verizon is the best carrier as far as making calls are concerned. You pay a premium for just their service alone, but it is well worth it and I can usually always take/make calls. But don't get me started on how much they butcher their Handsets to milk you for all the money you have (Ringtones, Ringback, VZCast etc.)
Sprint, I think is the cheapest, which also means they are one of the worst services...Yeah, I like the logic in that...
Umm other than that, all over ones are pretty much on par.
BTW, AT&T and Cingular are the same now...AT&T