Tracy Ulman whenever she plays an American. Once she was a Brooklynite, and had the class not to oversell it.
i’d go for frances macdermott in fargo, closely followed by mike myers, fat bastard - not a precise recreation but funny as hell
Those aren’t Wisconsin accents in Fargo-they’re Minnesotan, dontyaknow.
The accents were good, with the exception of Steve Buscemi, who didn’t even try.
Christopher Guest as “Harlan Pepper” in Best in Show
AND
as “Corky St. Clair” in Waiting for Guffman.
Oh, yes! And my namesake to boot! I dislike most movies, but that one was great. 
…
While we’re talking “accents” here, may I hijack this thread just a tad? OK… so who thinks that people who live along the shores of the Great Lakes between Buffalo and Chicago (heck, maybe it’s beyond Chicago; I’m not sure) have very pronounced accents that most people don’t even know exist?