I’d pick an airline you have a chance of using the miles for an upgrade on, and ideally one you can also transfer credit card miles to so you can top up for a reward.

TATL, Virgin Atlantic would be a good choice. High business fares earn 400% miles, which is pretty great, and upgrades on their flights are a nice value from premium economy.

TPAC, I’d probably go Korean Air; the earnings are lower, but the miles are pretty useful. They are one of the few SkyTeam airlines with an international first class worth redeeming for, and they have credit card transfer partners.

Booking class and mileage distance of flight would help us guide you more precisely; money is irrelevant when crediting DL flights to partners.

I would probably steer far away from FlyingBlue (AF/KL); their new program is pretty stingy all around, as I understand it...