CREATE TABLE table_203_153 (id DECIMAL, "stamp issue date" TEXT, "stamp set" TEXT, "#cards" TEXT, "card set ref" TEXT, "qty. issued" DECIMAL)

which stamp set had the greatest quantity issued ?
SELECT "stamp set" FROM table_203_153 ORDER BY "qty. issued" DESC LIMIT 1
SELECT "stamp set" FROM "table_203_153" ORDER BY "qty. issued" DESC NULLS LAST LIMIT 1