What are the most challenging or clustered major US airports to flying into (pilot / passenger observations)?

In response to the accident at DCA, I want to ask what major airports (ie not GA airports) have reputations of being challenging whether it be: - short runways - busy airspace or congested - difficult ATC - strange approaches - propensity for difficult exterior conditions (altitude, weather, etc.) - proximity to major population centers / buildings - other

I know the accident near Reagan Airport was a mid air collision where the helicopter was likely primarily at fault but it’s also come to my attention that DCA is considered one of the more challenging airports because of the airspace, being switched runways at times, extreme pressure on approaches due to capacity, river visual, etc. put differently the conditions around DCA could have been contributing factors.

So what airports have reputation among professional pilots of being the “more challenging” ones in the USA? Maybe some airline pilots on here can jump in if there are consensus ones in the community and maybe do airlines staff those routes with more experienced crew?