I'd vote for the South Florida Rainbow Snake
(Farancia erytrogramma seminola). Last one seen in the field was 1952.
Florida State Museum says it may be extinct.
Just a note about the word "rare".
To some it means rarely found by herpers.
To some it means in need of protection because populations are small and imperiled.
To some it means, if I can call it that, I can sell it for more money.
To some it means it hasn't been put on the grill long enough.
The South Florida Rainbow qualifies for the first two.
Edit
--- Guess I didn't see
NORTHeast

........ Don't get old... It's not worth it...
Of course, I guess the South Florida Rainbow is even rarer in the Northeast!!
This would be a tough call since so many ranges peter out as you travel north and east, and often their habitat requirements make them scarce in one area and common in another.
I grew up in the NE appalchians. On my mountain, Black rats were non existant, yet only 15 miles from me they were common.