Great question. I live in MD suburbs. 30 min to DCA, 30 min to BWI, 60 min to IAD. I am all in on United (that's all I fly). I fly a lot (1K status for last decade). I am probably 60% DCA / 30% IAD / 10% BWI

I hate IAD because of long commute (for me) and long walk/train/and-more-walk from parking garage to gate. I timed it and it takes me 25 minutes from park to gate at IAD (that is with Clear and TSA PreCheck). DCA (and BWI) have much shorter walk times once you park. So I'll usually accept having a connection (from DCA/BWI) over a non-stop (from IAD). I pick IAD for SFO or LAX (lie flat seats), destinations on the Eastern seaboard or international. For example I flew to CLT last week and would rather fly IAD-CLT over DCA-EWR-CLT. I am flying with OPM so cost is not a factor.

I usually pick DCA over BWI (equidistant for me) because DCA has United Club (albeit the most archaic outdated club in the network) and usually has better flight selections (i.e., times).