CREATE TABLE table_203_506 (id DECIMAL, "opponent" TEXT, "played" DECIMAL, "won" DECIMAL, "lost" DECIMAL, "drawn" DECIMAL, "% won overall" TEXT)

who did they have the largest win/loss differential against ?
SELECT "opponent" FROM table_203_506 ORDER BY ABS("won" - "lost") DESC LIMIT 1
SELECT "opponent" FROM "table_203_506" ORDER BY ABS("won" - "lost") DESC NULLS LAST LIMIT 1