Had AT&T for about 8 years until February of this year. Switched to t-mobile in Feb, then to Verizon with the release of iPhone6. At work was Nextel, then Sprint then Verizon.
AT&T was ok, but away from populated suburbs, the coverage isn't good. Data rates drop dramatically or are nonexistant, even if voice is still active. This can include Interstate highways between lower population cities.
Verizon seems to have a median amount of coverage *everywhere* I go.
T-Mobile is great in a major city, or its close suburbs. If you don't travel much, this may not be bad. If you do travel, likely gonna be SOL unless its to another major metropolitan area. Overall, I was pleased, but only with the cost of the network factored in.
Its been a bit since I was on the Sprint network, they have rebuilt and changed a lot. But it wasn't good then, if it was me, I would buy T-Mobile service.
I ended up on Verizon after travelling with my personal AT&T and T-Mobile phones, comparing it to my work issued Verizon. I felt VZW was the best bang for the buck, across the board. Look up the Verizon ads where they show the coverage maps between the carriers. Having used them all, I think it was an accurate and brilliant way to market.