When roxanne and daniel are the couple what is the highest rank?
CREATE TABLE "table1_19744915_18" (
  "rank" real,
  "couple" text,
  "judges" real,
  "public" real,
  "total" real,
  "vote_percentage" text,
  "result" text
);
SELECT MAX("rank") FROM "table1_19744915_18" WHERE "couple"='Roxanne and Daniel';