I have status through AmEx, which makes me a bit agnostic between DL, US and CO (through Sept). But I'll fly WN if they're the only direct flight (Denver, Las Vegas from RDU) and UA or AA if I'm paying and they're substantially less expensive (which never happens anymore).

That said, I've flown Delta 90% percent of the time in the last year. Their options on my specific routes, their domestic award availability and their constant top-notch mile-inflating promotions through AmEx work for me, although I understand the frustration of the non-AmEx or primarily international flyer.