What is the highest numbered nightly rank for any episode? 
CREATE TABLE "australian_ratings" (
  "order" real,
  "episode" text,
  "original_airdate" text,
  "timeslot" text,
  "viewers_100000s" text,
  "nightly_rank" real,
  "weekly_rank" real
);
SELECT MAX("nightly_rank") FROM "australian_ratings";