The Starwood Preferred Guest Amex transfers points 1:1 with Delta and will give you a 5,000 bonus of miles every time you transfer 20,000 points. SPG also transfers to over 30 other airlines. The annual fee is $95.
Another option would be an Amex that earns Membership Rewards that transfer to Delta. The Amex Everyday (no fee) and Amex Eveyrday Preferred ($95 annual fee) transfer MR points to Delta at 1000 MR for 750 Delta Points. While the transfer rate is poorer the earning rate is faster due to the bonus categories - Rewards: 3x groceries, up to $6,000 per, 2x gas stations, and 1x everywhere else. In addition, earn 50% bonus on purchases if you use your card 30 or more times in a statement period. Note that the 50% bonus works on both base points as well as category bonus points.