For awhile I used an old iPhone 4s a friend gave me when he upgraded. Since Verizon unlocked all those older phones years ago I switched to a FreedomPop account that cost $50 a year for unlimited talk/text and 1 GB data per month. Because it's VOIP some of that data was needed for talk and text. But it was still enough for my modest needs. And it includes reasonably accurate voicemail to text translation, so I don't actually need to check my voicemail to see who called and whether to call back.
That particular account has been on AT&T for a couple of years, same coverage as any AT&T service. You can buy the SIM and plan info at Best Buy and other stores, then activate online. Works with any compatible unlocked phone.
But eventually I needed more data and more convenient cell service. The VOIP type service with FreedomPop was glitchy with the old iPhone. So I got a Moto E4 and used it with Verizon until it was eligible to be unlocked. Since then it's been on Mint Mobile which uses T-Mobile service. It's not perfect but good for what it costs, around $25/month with unlimited talk/text and 3 GB data per month. My only real gripe is voicemails are often delayed up to 30 minutes, and the voicemails aren't date/time stamped, and there's no voice to text translation. But it gets the same coverage as T-Mobile and good data speed.
I think the Moto E4 has been discontinued but the E4+ and E5 are among the best values in Android phones. Reasonably fast processors and features for the money. Some include sensors that can be used as fingerprint ID and/or scrolling. Had mine for over 18 months, no complaints for the money -- $50 plus a 3-month commitment to Verizon, after which they unlocked the phone and I switched to a cheaper service.
If I upgraded it would mainly be for more memory. Turns out 16 GB internal isn't quite enough for all the apps I run. And while there's a media card slot with a 16 GB card I added, it's of limited use. It's only useful for storing photos and videos and a few apps. But the apps run sluggishly from the added card, and the OS can't recognize it as a seamless storage device, so I need to go through a kludge method of transferring photos/videos back and forth. So a 32 or 64 GB device would better suit me since I take lots of photos and videos, and use lots of resource intensive apps, but no games.