this discisstion in the past elsewhere has brought up two possible answers. depth from the street or depth from sea level. Roosevelt Island is not as deep as 191st from street level, but is bellow the level of 191st becuase 191st street level is higher.
Highest: I would have guessed Gun Hill Rd on the 2, or along the last elevated in Manhattan on the West Side (is that the 1 line?). Not sure if there's a station on that stretch.