It depends on how often you fly, your spending habits, and whether you are loyal to one particular airline. Thus, when you do not fly a lot, having an airline branded or travel tailored credit card won't have sense. You won't simply earn those big rewards in travel category if you do not travel. A travel rewards credit card that gives equal rewards on all purchases will be a better choice. For example, a best for travel credit card can be the Discover it® Miles card. It can become the best choice if you do not want to pay an annual fee. This card will earn you 1.5x miles per dollar spent on purchases, and the sign-up bonus is easy to get. You don't actually have to meet any requirements. You just use your card for purchases as you always do and then, when first year of cardmembership ends, get all earned miles matched (for new cardmembers only).
If you are a bit of a traveler, but do not want an airline credit card, you may like a credit card that earns increased rewards on travel expenses and gas purchases. The Citi ThankYou® Premier Card may become your ideal travel partner. It gives 3x points on travel (including gas), 2x points on dining out and entertainment, and 1x points on all other purchases. The sign-up bonus, as well as travel benefits (remember this card is designed for travelers) are also present. All those perks do not come for free, there is an annual fee of $95 for the card (waived first year).
Frequent flyers who are members of a frequent flyer program and prefer flying with one airline, will want to have an airline credit card that will earn them frequent flyer miles. For example, those who fly with American Airlines, may find the Citi® / AAdvantage® Platinum Select® World Elite™ Mastercard® (expired offer) credit card quite rewarding. Significant part of miles rewards is earned on American Airlines purchases, at gas stations and restaurants – double AAdvantage miles per dollar spent. All other purchases will add up only one AAdvantage mile. The card has travel benefits like free checked bag, and of course, it's not free – cardholders pay a $99 annual fee (waived for the first year).