Depends what you’re looking for.
For a while, Washtenaw county Michigan was all over the place.
1896-1960 (except 1912) Republican
1964-Johnson
1968-Nixon
1972-McGovern
1976-Ford
1980-Carter
1984-Reagan
1988-Dukakis
It’s been Dem since ‘88.
I mean if you think about it's not
that shocking and can at least be logically explained. Say it's a very liberal, college-ish county and narrowly backed McGovern after narrowly supporting Nixon. It's possible progressives and college students don't voe in 1976, and some flip to back native son Gerald Ford, so the county flips Republican (but narrowly). Then in 1980, Reagan isn't from Michigan, is much more conservative than Ford, and nearly derailed Ford in the '76 primaries, so he's obviously not a great fit for the county, so it narrowly flips back Democratic. In 1984 it's swept up in the GOP landslide and flips Republican, by 1988 the climate is slightly better for the Democrats and it flips Democratic, and it remains so thereafter. Though I agree that regardless, it's crazy to find a Nixon/McGovern/Ford/Carter county.
As of right now, I'd say Kenedy County, TX. It has the longest losing voting streak of any county - as the only Romney/Clinton/Trump county in the nation.