Grab the AA Citibank card if you can get the 10K with 6 months free. More info on the AA board (including a link, I believe). That's a no-brainer.

After that, it depends on your level of spending. If you don't spend a lot on a credit card, you might be better off with a no-fee cashback card or one affiliated with your favorite gas station. For low spending, quick small rewards are probably better than FF miles.

(If you can't get that AA card for free, search the AA board anyway - the folks there will give you lots of ideas to get the 5,000 miles you need for a free ticket without a big cash outlay.)