I favor cash back rewards over other rewards, because I can use the cash to fill an immediate business need. Most other rewards can only be redeemed for very specific items. I've seen two types of cash back programs:
- Earn X points, and redeem for Y dollars back
- Get X% cash back every month
I prefer the second option, because again I'm getting the cash back immediately instead of having to wait until I accumulate some number of points.
I like the American Express SimplyCash card. The only negative is that American Express is not as widely accepted as Mastercard and Visa, because, as Frank pointed out, they have the highest merchant fees. So if you get an AMEX credit card you may want to also get a Visa or MC.
I always make sure to pay off the full amount every month. Note that credit cards with rewards programs tend to have higher interest rates than those that don't. If you are not paying off the full balance of the cards every month, you'll end up paying more in fees than you get back in rewards. Those that are in this situation should look for credit cards with low interest rates, and forget about the rewards.
Also, look for a credit card with no annual fee.