What's the Best Prepaid GSM carrier in the USA?

I have an unlocked iPhone 5 so i can use any GSM carrier. What's the best one? Please no CDMA carrier

Well there are only two companies that have GSM networks and uses sim cards here in the United States. A T&T and T-Mobile. A T&T has more coverage than T-Mobile and less dead spots. T-Mobile has a lot of dead spots and not great coverage in certain cities. So A T&T would be a better choice.