Verizon. Been with them for many many many years at this point.
Other companies are garbage in comparison (in terms of coverage).
This entirely depends on where you live. I live in a decent sized city with lots of tech workers, and Verizon doesn't get signal at my house at all. I once went to a state where the only provider with decent coverage was Sprint, of all things (this was before T-Mobile bought them). I've switched providers after moving a couple of times due to that. All three of the big providers are pretty even in overall coverage, so you need to check with people in your area to see who has the best coverage near you.
I have T-Mobile and really like it. I've used both AT&T and Verizon in the past.
I pay $80 (including taxes and fees) a month for two lines with unlimited everything. I got the plan before 5g existed, and the plan was discontinued before it came out, but they still gave me unlimited 5g data once that became a thing. Other providers used that as an excuse to get people off of grandfathered plans. I was also able to buy an add-on online for international coverage for $15 a month. Was super easy, again let me do it on a discontinued plan, and I get unlimited texting and 5GB of 5g data in almost every country. I can just as easily turn it back off when I don't need it anymore. The base plan includes service in Canada and Mexico, but with some limits IIRC. Other providers may be similar here, I've never traveled internationally with any other provider.
When I had AT&T and wanted to stick with a cheap, discontinued plan, they did everything they could to get me off of it. I knew somebody on Verizon who couldn't get a new phone unless they got off their discontinued plan. T-Mobile goes out of their way to make sure I can keep my plan if I'm happy with it (which I am).
I buy unlocked phones directly through manufacturers instead of upgrading through my provider (providers often add a custom OS with extra junk and delayed updates), so I can't comment on that. I don't even tell T-Mobile I'm getting a new phone. I want to say when I did that with Verizon, they charged me a fee to activate the new phone. I remember Verizon charging me a lot of fees. That's never happened with T-Mobile.
I've also had much better customer service from T-Mobile than Verizon. I always had to wait for ages whenever I went into the store for anything. I liked AT&T customer service, they were always friendly and helpful, but it's been a long time since I was with them.
I personally wouldn't ever to back to Verizon, to the point where I wouldn't move to an area that only had reliable coverage by them. I liked AT&T mostly fine, I only switched away for practical reasons, and I regretted it. My experience with them was from quite a while ago though.
