We're AT&T and it works best for us. They changed systems a few years back and their coverage suffered for awhile until they built more towers. All-in-all, it's good now.
We did have a lot of billing issues with them for awhile, but that seems to have straightened out too. We have a corporate account for the business, and they jump through hoops for us, give us free stuff, etc.
If data speed is important to you, At&T has an edge there over Verizon. And AT&T uses GMS system which utilizes sim cards in most phones. I like being able to switch phones just by swapping the cards. You can't do that on Verizon.
Verizon probably has the best overall coverage in the country. And they seem to have the broadest selection of phones.
T Mobile is very good in most areas. I'm disappointed that AT&T wasn't able to merge with them, I think it would have been good. And they use the same GSM system.
Sprint/Nextel is OK. Nextel ticked me off years ago, and Sprint's coverage is not quite what the others are. But they do have very fast data speeds.
Really it just depends where you are and need to use your phone. All of these have areas, especially in Paulding, where their service is poor. Oddly, Verizon is probably worst of the four in Paulding, even though they might be the best nation wide.
I think they will all give you a 30 day trial. I would definitely utilize that before getting locked in.
You should probably factor in who you talk to the most and what provider they use. All AT&T calls, for example, are in-network to any other AT&T cell phone, meaning they don't go to your minutes. And you can pay a small fee and make AT&T landlines in-network. I think the others have similar "in-network" plans.
And then there is bundling, which sometimes pays off and sometimes doesn't. You might can bundle your cell service with landline and internet, even TV programming. You just gotta run the numbers and compare.