Crews Hill Station
It's very close to the M25, and is the northernmost station on its line (the Hertford Loop Line) to be inside the TfL zonal system.
The station is unstaffed, but does have an Oyster reader and a ticket machine. As of August 2017 the ticket machine can only sell paper tickets — no Oyster top-ups.
It previously had evening and weekend services to King's Cross, but as of late 2017 this is no longer the case.