Lots of "best credit card" threads here...need more info to point you in the right direction, first and foremost would be whether you're looking for a U.S. or U.K. card. Second question would be what kind of rewards are you looking for? Quick but small cash or petrol rewards, free hotel stays, airline miles, home-grown points, etc. Do you need specialty features such as those you'd get from Diner's or one of the super-premium Amexes?

In the U.S., I think most FT'ers look at the Starwood Amex as the "gold standard" for rewards cards. Any comparison of cards usually includes that one or the HHonors Amex. But it doesn't really matter if you are dead set on some other particular type of reward.