Right now there are essentially three carriers. AT&T, Verizon, and T Mobile. T Mobile bought Sprint and now owns all of the Sprint cell sites. At some point, Sprint users will probably have to buy new phones as T Mobile updates the former Sprint sites.
Everyone of the discount companies, Metro, H2O, Cricket, Pure Talk, etc... buy air time from one of those three and then resell it along with mostly last generation phones.
In general, right now Verizon has the best overall coverage. Keep in mind that the paths along or near Interstates and other highways get priority for equipment upgrades since that's where most users are.
Note that my comment about Verizon reflects my area, your area may be different. Last I knew, my son had AT&T because his employer pays for his phone and they decide who the carrier is. He hates it. Sprint was worse, and Verizon was the best. Again, that's very often location based.
As to phones, I have a 5 year old Galaxy Note 5 because it's all I need. If I need to do heavy duty computing, I'll use my laptop if I'm on the road and desktop at home.