First of all, that's not the Golden Rule. The Golden Rule is a precious truth which should not be replaced with aluminum.
T-Mobile doesn't have terrible overall coverage by any means. However, there are spots in the country where you can only receive a usable signal from one carrier (or none at all). I've been in spots where T-Mobile is that carrier. But if it's very rural there's a better chance it's Verizon. Verizon's more widely spaced antennas give it more coverage area, but also more chance of congestion. Every single carrier has given me times when I've been elated with them over all the others, and times when I'd almost rather smash their device on the ground.
Yes, sometimes I think AT&T and Verizon are up in the stratosphere, because all that capacity must be transmitting somewhere! I'm sure if I were their only customer, and I didn't have to share, they'd be the best.