A few small planes, maybe not the smallest on the list here:
DeHavilland Twin Otter between Syracuse and Watertown, around 1977.
Britten-Norman Islander between Kirkwall and Papa Westray in the Orkneys (yes, we have the certificate for flying the world's shortest scheduled route between
Westray and Papa Westray

)
DC-3 between San Juan PR and St. Thomas, USVI, 1980. Not that small but remarkable that it was still in operation.