There are only three carriers(maybe 4....I can't keep up with all the mergers) with actual wireless networks of their own: AT&T, Verizon, and T Mobile (and maybe Sprint, which has either or is in the process of merging with T mobile). Everyone else is a reseller. By far the largest resale network is the one run by T-Mobile. So, when you see all these smaller companies and want to know how their service will work for you, look at what T-Mobile offers in your area.
I have T-Mobile for one of my phones, and it works well, but there are definitely small pockets in extremely rural areas of my state where I lose coverage. Not a big deal to me because I'm rarely there. For my other phone, I use Consumer Cellular, which offers very inexpensive flat rate service. They resell T-mobile, so (no surprise) coverage is equivalent).
All carriers (at least in my state) offer a risk free trial period so you can figure out in actual practice whether their service is acceptable in your area. If you are like most people and live in a reasonably urban or suburban area, you can pretty much safely go with any carrier. If you live more rural, or NEED your phone in more rural areas, then Verizon or AT&T are your best bet. Also, be aware that your cell phone can be locked to a specific carrier, and you may NOT be able to port your phone over to new carrier (you'll have to get a new one). This is a cost you will need to factor into your analysis.