T-Mobile USA, I have been with them since I was 14 years old, and I am now 22. Coverage hasn't ever been an issue for me, but EDGE is getting really long in the tooth (thankfully TMO is fixing that by the end of this year). I always buy my phones outright anyway (Just like my iP4 I am using now), so whether T-Mobile "officially" carrying the iPhone is meh to me. I plan to buy the iP5 outright as soon as it comes out to continue using it on T-Mobile.
The reason? Nobody has yet to beat my plan:
-Unlimited Minutes
-Unlimited Data
-Unlimited Text Messages
-On 4 iPhones for a price of $162
I have had official iPhones on AT&T and VZW as well, as secondary lines (because I was craving 3G) but it was not worth me spending the money. Nothing wrong with either AT&T or VZW, just didn't want to keep spending money I didn't need to spend. Also didn't notice any coverage gains with either of those carriers, then again, I don't travel to rural areas quite often enough.
Never tried Sprint yet however.