Well, I'm not the most frequent flyer by any stretch, but let's see...
Of course, all NW domestic hubs, plus BHM, MOB, PNS, GSP, CLT, LGA, DCA, PVD, SEA, LAS, FLL, MCI, plus I've flown NW to/from AMS & FRA. (This doesn't include some cities flown to/from on NW predecessor airlines Southern Airways and Republic Airlines).