I've had good experience so far with Cycle City in Sloatsburg, NY (Rockland County). It's accessible via Metro-North from NYC (Port Jervis line, 20 min walk or 5 min drive from the Sloatsburg train station). They have my bike right now for a warranty repair, in fact.
Avoid Brooklyn Moto in Greenpoint. They sold me my Enfield, but the service department has treated me poorly since, and I don't think they're officially authorized.
The Long Island dealers are respected but I don't know them.