All of the carriers except Verizon offer some sort of unlimited calling option with their introductory plan. AT&T and Sprint have mobile to ANY mobile and T-Mobile's unlimited plan cost nearly the same and they even have unlimited WiFi calling. For me, that tosses Verizon right out of the Window.
AT&T of course has beaten to death because it was the only carrier to offer the iPhone and people had to have the iPhone despise the fact that AT&T had limited coverage in their areas. (Every carrier will have their weak areas) AT&T also uses GSM technology which means you can do voice and data simultaneously on your phone. You'll also have much faster 3G speeds as they have now implemented HSPA+. You have theoretical download speeds of 21Mbps. The next iPhone is VERY LIKELY TO SUPPORT HSPA+.
Verizon definitely has the most reliable network but they use CDMA technology which is older and not used throughout the entire world. If you travel a lot you will have issues. They have a very slow 3G network as the theoretical max is 2.4Mbps so you'll see much slower speeds. Now of course, that won't matter if the next iPhone is LTE but I say what's the point with their extremely expensive tiered data plans. $30 for 2GB is quite a lot and AT&T is only $25 for 2GB. That's a difference of $120 throughout the life of the contract. I'd go to Verizon if you can get a huge discount through your employer or school.
Sprint is slowly increasing their prices to that of AT&T and Verizon. Most people don't even realize this and Sprint really isn't the best option anymore unless you have to have their unlimited data plan. Keep in mind, its unlimited because its so slow. You will be lucky to get 1Mbps in most places. I say Sprint's 3G network is the equivalent to AT&T's EDGE (2G) network. As with Verizon, Sprint uses CDMA technology so you'll have extremely slow data speeds. Apple will NOT support WiMax only for Sprint.
T-Mobile has a really good network but limited coverage and the fate of that carrier is unknown which is probably why Apple passed them up for the iPhone. They use GSM technology like AT&T and will have much faster speeds but those who use an UNLOCKED iPhone on their network will only get 2G speeds because they use different frequencies than AT&T. T-Mobile's EDGE network is extremely slow and way worse than Sprint's 3G.
I'd say AT&T is the best bet and if they have limited coverage you could always invest in a microcell which is an in house cell tower.
