It invariably happens: Your mobile phone connection is dropped during an important discussion; you can't check those maps when you're out in the boondocks; you need something fast but your connection is slow and the download stops and starts. So, you start looking for a new provider, and now it's time to pick through all their claims to find the truth.
The truth is the big four carriers (AT&T, Verizon, T-Mobile, and Sprint) are pretty equal in most respects. US Cellular has its own network of cell towers also. You can look at smaller and regional carriers, but those usually piggyback on one of the big four as an MVNO (Mobile Virtual Network Operator). For example, MetroPCS uses T-Mobile's towers, Boost and Virgin use Sprint's network, Comcast's new Xfinity Wireless runs on Verizon, and a lot of local carriers use AT&T. If you are curious what your carrier may be using, you can find a list here: List of United States mobile virtual network operators
When it comes to speed, CIO Today says AT&T, Verizon and T-Mobile are tied at the top. (See Verizon, T-Mobile, AT&T Tied in Network Speed Tests) Sprint cites reports saying they are within 1% of the speed of the others. Mileage varies with all of them, of course, based on where exactly you are.
And that is it in a nutshell. The question isn't really which carrier is the fastest or most reliable or "The Best," but which one is best for your needs and your area. Your best bet is to ask family, friends, and people in your area how their experience is. Then take into account where you go the most. For example, here in our SoonerSoft offices, our T-Mobile users get great coverage around the cities and larger communities, but out in between cities in the more open spaces of Oklahoma away from major highways they end up roaming (no charge usually) or losing connection entirely. AT&T and Verizon users generally get some connections out in the rural areas, and Sprint is spotty even in the suburbs. However, in other states, Sprint can be just fine. Then, compare how you use your phone to them. Do you make more phone calls or use texting more? Do you listen to a lot of music or do you use the internet browser and Facebook more? Different apps will act differently on each network in each area.
We know it isn't a very definitive answer, but it's the best one we can give: ask your friends and check out the connections on their phones as you decide on a carrier that works for you.
Lastly, don't forget all of the providers have various deals to try to get you to choose them. As an example, T-Mobile not only offers four lines at $35 each, but they also have many music and video services that don't count against your data usage. Streaming a lot of music and video just won't go over your limit as much as it could on AT&T or the others. They also roll taxes and fees into the base charge -- no extra charges on your bill that way. (Other carriers are starting to do this, also; isn't competition great?) Don't have four phones to get that low rate? You can check out MetroPCS, US Cellular and some others who often give lower monthly charges with full plans. Just keep in mind the carrier network they use and how that will affect your connections.
Whichever carrier you choose, you may want to make sure your phone is unlocked so that you can move easily to another carrier when you need to. Your carrier can perform an unlock for you (usually they have some conditions and fees). Or, you can purchase an unlocked phone and be prepared whenever and wherever you might go.
It may take a little work, but with just a little asking around, you can pick a carrier that hopefully will fit your needs for several years!