Probably Virgin Mobile. They are aggressively marketing unlimited text/data plans for the prepaid market. But they have two huge weaknesses.
1)No Bring Your Own Device. Something many if not all Verizon resellers allow.
2)They only have Sprint native coverage with no roaming.
(OK, 12 days later ...)
That, and I've heard the customer service is horrendous for most prepaid providers, including Virgin. It can take several days to port a phone number, and it can at times be difficult to get through to a CSR when you need help with anything. But I suppose, if you're a first-time cell phone owner, you wouldn't know what you were missing.
But, yeah, Virgin does a good job on marketing to that demographic.