Depends how you're going to use them, too.

The WalMart/AT&T ones have HUGE fees for using pay phones, and other surcharges.

The Costco MCI cards are a great buy and also have one of the lowest charge for using a pay phone.

I found a site that lists all of the brands of cards and their surcharges - sorry I don't have the URL, but do a google search and you should be able to find it.