CardWeb presents a redemption value study of a number of credit card reward programs. The result:
Credit Card (Minimum to redeem), Value per Mile (Point)
Diners Club (25,000), 2.5 cents
American Express (20,000), 1.2-2.5 cents
Discover Miles (10,000), 1.0-2.2 cents
Fleet Miles Edge (18,000), 1.8-2.0 cents
Bank of America Travel Plus (10,000), 1.6 cents
MBNA WorldPoints (25,000), 1.6 cents
CapitalOne Go Miles (9,000), 1.1 cents
BankOne Travel Plus (5,000), 1.0 cents
To be sure, this is not an apple-to-apple comparison. I'm pretty familiar with the MBNA WorldPoints program because MBNA MasterCard is my preferred card in the last few months. The 1.6 cents quoted in the study is the maximum value of a domestic round-trip ticket ($400). Anything above that price needs to be paid by the customer.
On the other hand, some other programs offer more flexibility. As noted in another CardWeb article, CapitalOne Go Miles does not ask a flat number of points in exchange for a round-trip ticket. Instead, you can buy whatever ticket you need and redeem points at a solid 1.1 cents/point rate. (The CapitalOne card has $19 annual fee attached, though.)