Interesting thread: I don't transit the US as much as I did pre-Covid, but my 2 cents... (Obviously if we were including D1 lounges those would win every time - I purposefully transit through LAX now for that reason alone)
1) LAX: the dual serving areas help with crowding & staff seems to be much more professional, albeit in an LA-esque way
2) SEA: generally good, open layout compensates crowding (which is on-off depending on time of day), staff has been helpful when needed
3) BOS: the new lounge has been pretty good, but I've only been there a few times lately
4 tie) ATL D: perhaps only on the list by comparison to the other ATL clubs, which go from dire to abysmal to "better off in the terminal"
4 tie) MSP [by the G gates]: only twice in the last year but seemed much improved

On the other hand, I will do just about anything to avoid DTW (for the lounges among much else) and have had spotty experiences in SLC (new terminal is good and the staff are generally among the better in the DL ecosystem, but the lounge has always been crammed full). I liked AUS pre-Covid but haven't had experiences with it since then. JFK is... JFK.