If you're willing to commit to Delta, the Delta Platinum SkyMiles card is the way to go. After signing up for a Delta SkyMiles frequent flyer account, apply for the Platinum card.

Right off the bat you'll get 15,000 miles. For each Delta ticket, most gas, supermarket and drugstore charges will net you 2 miles for each dollar. If you charge more than $25,000 annually, you will get a bonus of 10,000 miles which count towards elite qualification. After $100,000, I believe you are limited to 1 mile per dollar. On your anniversary date they will give you a free companion ticket certificate for travel in the U.S., although some have said that it is of limited value.

Each Amex mileage posting counts as activity in your Delta account so, if you never fly on a paid Delta ticket, your miles won't expire as long as you keep charging.