I would prefer T-Mobile due to their better and more down-to-Earth Plans, but their 3G service is not compatible with the iPhone 5 (different frequency); only EDGE works with the current iPhones. (Apple, please support T-Mobile's 3G!!!).
I would only choose AT&T because they are the only ones carrying it in GSM. But their plans are ripoffs.
Wouldn't choose Verizon because they only support CDMA, and never Sprint as their customer service is terrible.