AT&T - decent for US travel, but you'll pay for it
Verizon - again for travel, but alot of hidden fees and costly if everyone you know doesn't have it.
Sprint - STAY AWAY
T-Mobile - stay away from the MyFaves, but otherwise these guys are great (also, double check the coverage zones, they merged tower use with Cellular One and cover just about everywhere) I've had a family plan with 4 phones with them for 5 years now
AllTel - not as nice as the commercials make it, also do not get if you travel
Cellular One - best plan for teenagers, get them a local area with unlimited long distance etc, and disable roaming. As long as they're within 50 miles of home, the rugrats can't screw up your bills
Qwest - Hell No