A veritable cottage industry has formed in recent years around travel hacking, mostly focusing on how to accumulate as many miles as possible for airline reward redemption. The whales of these strategies are the giant mileage bonuses offered by bank and co-branded airline credit cards.
But, given the depth of card choice, which card is the most rewarding?
After combing through the deep, dark depths of the reward cards' terms of service, IdeaWorks, in Switchfly-sponsored research, has come to the following conclusion based on some serious calculator jockeying:

American Express, Barclays, Chase, Citibank and Southwest are the most rewarding. American, Delta, US Airways and United cards offer travel perks but higher reward thresholds.
Here's the breakdown of the average value of these rewards credit cards:
As you can see, there are some very clear differences in mileage rewards delivered by the various cards - especially after considering any sign-up bonuses offered.
These differences show very clearly that the co-branded cards are better for frequent flyers looking to redeem international premium travel, while the bank rewards cards are ideal for mostly domestic travel.
When considering the average domestic flight round-trip cost of $367 (third quarter 2012), the redemption value of the bank rewards cards allows faster purchase for those seeking domestic travel. Chase Sapphire offers over 2 domestic ticket rewards, and even the lowest value card, the AMEX Blue Sky, provides the equivalent of 1.5 domestic flights.
When considering international premium travel, the picture changes drastically as the airline co-branded cards really show their value:

The price of a business class ticket on American Airlines between New York and London is $3,521 (fees and taxes included) for a query made on 01 February 2013 for a March 1st departure and a March 7th return. The very same flights can be booked using 200,000 AAdvantage miles (Business AAnytime reward) and paying taxes and fees of $342. This places a value of $3,228 ($3,521 less $342) on the 200,000 miles. An airline co-branded cardholder would spend about $200,000 to accrue that many miles (not including any sign up bonus). That same $200,000 charged by a Citi ThankYou Premier cardholder could generate travel reward value of about $2,800. This example relies upon approximate mileage and point accrual for both cards. But it does significantly favor the reward value provided by the airline co-branded card.
The value equation dramatically improves if American’s 100,000 roundtrip Business MileSAAver reward is available for the same itinerary, as it places the $3,228 value on 100,000 miles. This would require about $100,000 of charge activity by an airline co-branded cardholder (again, without considering a sign up bonus). This level of spending would provide a reward travel benefit of $1,500 from the Citi ThankYou Premier card. Under this scenario, the airline co-branded card delivers more than twice the value of the bank travel reward card. Banks are limited by the simple economics of the credit card business. They can only spend a finite amount of portfolio revenue to buy rewards. Airlines can always win at this game because air travel can be provided at low incremental expense.
So for those looking primarily for domestic tickets, a bank rewards card - cash back or travel rewards - will generally provide quicker rewards.
Internationally-bound travelers seeking a premium seat would be better served by dropping all of their spending on a co-branded airline card and then converting that reward for that international premium reward.
As always, it comes down to spending behavior - and what the cardmember intends to use the accrued miles/points for.
Download the full report here.