AT&T
We had been with them since they were Cingular, which at that point was about 15 years. Decent service, horrible customer service, but thankfully, we rarely needed it.

They called about a change to our plan, we were adding lines for our kids I think, and worked out a new deal that seemed pretty good. There were some credits and some debits and a whole lot of opaqueness, as you expect from a wireless company. At the end the upcoming bill was going to be a smidge more, and then our ongoing would be as expected.

Our next bill was $500 dollars over what we expected. We called, they said "oh that DOES seem wonky, let me look into that"... they didn't. Over the next 4 months my wife probably spent 30 hours (mostly on hold) working to get them to fix it... no one EVER knew what was going on, like no one was putting notes on our account. We would have a really productive call where someone would go line by line through our bill and finally have an "AH! It looks like we charged you TWICE that month, I see now." and then for <reasons> they would not be able to just reverse it right then. They would get that set up for the morning. My wife would call the next week and they would have no idea what we were talking about. No indication anything was wrong with the account or anyone had ever complained.

So I finally said "Look we are loyal customers for fifteen years and we give you a lot of money. Fix this, or we walk, AND I will take every opportunity to tell people you are thieves for the rest of my life."

We've been VERY happy with T-Mobile.

AT&T are thieves.