best oysters on the half shell?
The restaurant owned by MacMenamin's Grill in Larchmont. Before it closed.
MacMenamin's Grill in New Rochelle. But not for brunch, since they don't replace the oysters or clams after the first offering.
Gus's perhaps, in Harrison?
Other than that, I guess the Elm Street Oyster House in Greenwich -- but bring your bank book.