I went to WDW for all my life before DL and I had become so disenchanted with MK and EPCOT. I cannot realistically consider any of the other US parks in their current form superior to DL.
It depends on the last time you were at the MK. If you grew up going to WDW in the 90s and you have memories of the "true" MK and EPCOT Center, DL is very quaint but not much more than that. If you haven't been to WDW since the mid 2000s when maintenance was low, DL is incredibly better. But if you've been to the MK since NFL opened, it's a difficult comparison, because for all its faults, the MK looks really good right now (maintenance and overcrowding not withstanding).
So it becomes a comparison game.
Entrance: quaint and old-fashioned at DL, stunning at the MK
Main Street: feels like a place in DL and a concrete shopping center at the MK
AL: IJ is incredible but everything else is better in the MK (excluding POTC)
NOS vs. LS/CPlaza: POTC is better at DL and the HM is better at the MK. The ambiance is about equal, although I lived in the real New Orleans for a few months and NOS feels like a joke to me.
Fr.L: the entrance fort in DL is really cool because you feel like you're in a TV western from the 50s, and the ROA is a pleasant theme park stream; but the MK's FrL feels like an actual outpost on a real river. The MK Splash is much, much better than the dinky version in Critter Country.
FL: This is where things get complicated. IASW has a better façade in DL but superior staging and show scenes in the MK. The dark rides are better maintained in DL but significantly expanded in the MK (Pan is almost a full minute longer in Florida). DL has the upper hand with classic dark rides, the Matterhorn, and the canal boats; yet DL feels like a collection of unrelated rides compared to NFL, which creates an actual sense of place. Unfortunately, NFL contributes little more than pretty landscaping, a short coaster, a short dark ride, and a restaurant that actually requires FP for lunch! DL wins for attractions; the MK wins for landscaping.
Toontown/Storybook Circus: Storybook Circus is pretty enough, but ToonTown is better.
TL: Space is smooth in DL. That's it. TL is much better in Florida, which isn't saying much because both coasts need some futuristic help.
Castles: Sleeping Beauty Castle is a cute throwback to a simpler time in America. Cinderella Castle feels like a legit palace.
Overall ambiance: in its heyday, the MK felt like a true fairy-tale kingdom across a lagoon from reality. Now it's a corporate behemoth, with attractions ripped out for poor substitutes or nothing at all, and overcrowded streets bustling with tourists who are just trying to make it to all their appointments on time. DL is clearly a theme park in the middle of a city, and the pass holders wreak havoc on weekend crowd control, but it maintains a relaxed pace that feels like a vacation.
The choice is too difficult to make. I call it a tie.