When American Airlines ran a holiday promotion in conjunction with Mastercard on their joint venture SimplyMiles website, I went all-in. They offered sextuple miles on all offers, and the most lucrative was unlimited 40 miles per dollar donating to Conservation International – a charity Mastercard was raising money for.
That meant earning 240 miles per dollar, or buying miles at $0.0042 apiece. I did 7 million miles. That’s hardly ‘the most’ anyone went for. I know of several readers who did more than me. One reported earning 12 million miles with this promotion.