What is the best medicine or treatment for hay fever?

No one bullet drug: There is no "one size fits all" medicine. For mild symptoms, an oral non-sedating antihistamine may be enough (e.g. fexofenadine) You also should use HEPA filters at home and drive with closed windows. You should also rinse nose with saline. If this is not enough, a nasal steroid or antihistamine should be added. If insufficient, see a board-certified allergist to consider allergy shots.