To start with, there are two competing technologies CDMA (Sprint and Verizon, assuming you live in the USA) and GSM (Cingular and T-mobile). That might be a factor for you.
Then it depends what you want to do with the phone. A phone with a mp3-player and a camera might be a good deal, but if you only want it to phone, you pay way too much, so it's a bad deal for you.
Then it's not only the phone that counts, but the tariff (costs per minute, costs per SMS) even more, seen over the whole life (3 years, 6 years?) of the phone. A cheap phone with a carrier that has higher costs per minute may be a worse deal than the other way around.
A cheap subscription rate plan might be even be the best.
Then there are promotions and rebates that vary from state to state and from week to week.
And there's the difference between locked phones and unlocked ones (GSM only), but I don't think it matters much with prepaid.
The only conclusion: there's no single best deal for everybody. It's even doubtful if there's ONE clear best deal for you.
what prepaid cell phone is the best deal?