Hillingdon and Ickenham are the best options depending on the day of the week. Hillingdon is £6.50 on weekdays, £2.50 on Saturdays & Bank Holidays, and £1.60 on Sundays. Ickenham is £4.50, £2 and £1.60 respectively. So you’re better off at Ickenham Mon-Sat and Hillingdon (Hillingdon is closer to the A40) on Sundays.