What is the maximum ITV1 weekly ranking?
CREATE TABLE "series_1" (
  "num" real,
  "airdate" text,
  "no_of_couples" real,
  "first_couple" text,
  "second_couple" text,
  "third_couple" text,
  "fourth_couple" text,
  "viewers_millions" text,
  "itv1_weekly_ranking" real
);
SELECT MAX("itv1_weekly_ranking") FROM "series_1";