Judging from his marital history, an insertion sort.
I have AT&T. I live in San Francisco. AT&T regularly drops calls. I cannot make calls from home without dropping them a minute or two into the conversation. I could not make calls from work until they installed an expensive repeater. Notice that AT&T lost EVERY SINGLE reliability comparison.
For my needs, that makes them the worst provider.
Being an unfortunate customer of AT&T wireless, this means there is no acceptable tower anywhere.
I live in the middle of San Francisco, and AT&T usually provides no service where I work (North Beach), volunteer (Mission), and live (Castro). I once thought sprint was the worst mobile phone company ever but at least I got signal.