I have been on all 4 major networks in the last 5 years. TMobile was fine for me except. .. I started dating a girl in another town (20 minutes away not long distance ). There were a couple of areas where calling worked most of the time but data was nonexistent. Think about I got lost a few times going to work a strange way from her place and couldn't access maps. I hear they have improved a lot in the last 3 years or so.
Sprint was eh. But they don't like current customers only new ones. Their customer service was terrible telling you anything to get you off the phone.
AT&T and Verizon are about the same. Big enough they don't have to pander to customers, which is bad except that it offers some stability to an extent. They have great coverage with one or the other dominating some areas more than the other. Verizon seems to have better building penetration but a lot of that is somewhat balanced with the LTE bands. Your mileage may vary.
My wife swears AT&T doesn't reach inside her office so we're on Verizon. Personally I lean towards AT&T.
Now AT&T has WiFi calling like TMobile and AT&T just got approved to put it on iPhone. TMobile always did I think.
I'm looking at switching to T-Mobile as well. I've been a customer of Verizon (and predecessors) since 1998. From what I've seen with T-Mobile, they've expanded their coverage significantly over the last year. And they don't seem to be quite as money-hungry as Verizon (They can't afford to since they are the third largest carrier -- not the first). One factor in the switch is making sure your phone handles the bands that T-Mobile supports. I've been looking at the Moto X Pure edition because it supports all of the major carriers on the same phone. As far as coverage maps go, the Verizon ones are very inaccurate. It shows solid 4G coverage in places where 3G (or less) is only available. I've had more problems with calls lately with Verizon on multiple phones in my area, but no problems with a GSM TracFone that I have.