AT&T is expensive but very reliable. I currently have AT&T and I get signal 98% of the time. Inducing the outskirts of cities and rural areas.
T Mobile is cheaper but not as reliable in all places (rural areas)
Sprint is a tad bit cheaper than AT&T I think but it's good if you don't want data overages because they are the only phone company out of the 4 that offer unlimited data.
I'm not that sure about Verizon Wireless.
T-Mobile is the cheapest, but also has pretty much bad signal around a lot of places and more dropped calls anywhere you go out of the 4 major carriers. Data speeds are also pretty terrible around most places (unless you live in one of their high speed HSPA+ areas, which is rare unless you live in one of the bigger cities). Data can also be throttled if you're on one of their unlimited data plans. Their customer service however, is ok for the most part.... third place out of the 4 carriers.
Sprint is the second cheapest, but it has better signal than T-Mobile for the most part (of course, data speeds are pretty bad on Sprint). They still have unlimited data plans however, which will come in use when their 4G LTE starts rolling around to more places, which means faster data speeds as well. Their customer service is second place last I checked with the customer service satisfaction list.
AT&T is the second to last on most expensive, and their signal can be good (it's better than Sprint and T-Mobile), but you'll still expect dropped calls in a lot of areas. They have 4G LTE and it's pretty fast for the most part, but there's still a lot of areas that are still using the older HSPA+ technology instead. Their customer service however, is the worst I've ever had. And the surveys even reflect that as well. AT&T is rated as the worst carrier when it comes to customer service out of the 4 carriers.
Verizon is the most expensive, but they actually have signal almost everywhere you go (other than the mountains, which is the same with every carrier). Dropped calls are extremely rare unless you're in a dead zone. Plus, their 4G LTE network is pretty widespread at this point in time. To make it better, their customer service is the best out of the 4 as well.
Just make sure you speak with a technician and not with an assistant because technicians actually know what they're talking about when you speak to someone about your phone.