Not quite the longest, but the Crewe -> Manchester Piccadilly via the airport services all seem to have 15 minutes allocated at the airport. Which must be very tedious for people travelling from Styal to Manchester, as these are the only services they get, and means the journey time from Styal to Manchester is considerably longer than that to Crewe, despite Crewe being much further away *and* the latter having a 7-minute wait at Wilmslow.
(Not so bad in the other direction, with only 4 minutes at the airport).