CREATE TABLE table_204_155 (id DECIMAL, "rank" DECIMAL, "heat" DECIMAL, "athlete" TEXT, "nationality" TEXT, "time" DECIMAL, "notes" TEXT)

which country ranks the least ?
SELECT "nationality" FROM table_204_155 ORDER BY "rank" DESC LIMIT 1
SELECT "nationality" FROM "table_204_155" ORDER BY "rank" DESC NULLS LAST LIMIT 1