If you want rewards, SPG Amex is probably your best bet in general for forex purchases. Right now Amex Delta is the best with the double miles promo (this is the card I'm using right now).

But saying you'll only use a rewards card is IMHO very short sighted. You know the airlines are willing to sell you the miles at a grossly inflated 2.5c to 3c per mile. If you use Citi AA, Chase United, etc., you're paying 3 cents per dollar spent to get 1 mile per dollar. If you go with a 1% back card with a 1% forex fee, you're paying a net of 0 cents per dollar. So for the privilege of earning miles, you're paying an extra 3 cents to earn 1 mile. Maybe worth it if you need the miles immediately (due to fees they may charge you to purchase miles), but I certainly wouldn't do it.