Best pre-paid in US (or: why I'm not signing another contract).

Hey gang, what's the best pre-paid phone currently in the US? I'm thinking specifically of phones that allow one to use pre-paid service, not just phones that can be purchased off-contract.

In recent months, I've become extremely frustrated by the 2-year-contract system that is plaguing the American cell-phone industry. I just finished my 2 year stint with a Sprint Palm Pre (all jokes aside, I loved that phone), and logic would seem to indicate that an iPhone 4S would be the logical upgrade -- except when I look at the monthly commitment between on-contract Sprint (which is definitely the best deal of the big 4) and say, Virgin Mobile, I'm shocked at how much more one is charged for what is essentially the same service.

Also, I love an underdog.

Thoughts?