AT&T. I've been with them/Cingular for 10 years and never had an issue with them, I get data and voice (which is nice), I get to keep my unlimited (and I don't care there is a throttle point, I'll still get data and I won't get an overage charge), I don't care about facetime over 3G as I barely use it and I have only very rarely though if I could do facetime now (when no wi fi) I would, and they are a helluva lot cheaper since I for me the shared plans are a total rip off and even if Verizon offers the singular plans still (not sure if they do) they are still more expensive for my needs (they are cheaper if you buy a ton of data but for the data amount I'd pick they are more expensive).
Oh yeah, Sprint is not even an option. I had to deal with their crappy service when my parents had them, couldn't understand a word they said (at the time you didn't really see smart phones, that was like a little over 10 years ago). And this was the same for people in different areas of the country even so it wasn't just where my parents were. And from what I hear, maybe their voice is better but it sounds like their data isn't.