It's BNA, PIT (which IIRC has a nonstop to CDG on a 757), RDU or STL. For travel around Thanksgiving, there had been some extremely good I fare (discounted BE) deals from these cities, but availability is likely to be limited now. Remember that an I fare (or, in fact, anything M or above, so M/B/Y in expensive but upgradable coach or any business class fare) earns triple MQMs because the 50% MQM bonus for class of service is doubled too, and any BE fare (not including upgrades with miles or SWUs) gets 50K instead of 25K bonus miles from the four magic cities.

If you decide to do this, be sure to sign up first and read the rules on delta.dumb, the big FT thread, and the wiki referenced in the FT thread very carefully. It's better--and maybe cheaper--if the positioning flights are on separate tickets, but allow enough connecting time.

ADDED: If you do the promo from BNA, can you drive to that airport easily? If so, this would help to avoid problems with Thanksgiving time domestic availability and fares.