tubechallenge.com

As you can probably tell by now, I like to imagine new challenges, doing them... not so much.

This one... all the open stations that were once, but are no longer served by the Underground. I believe that means the following (please tell me if there are any more).

Once Bakerloo (now Overground): Headstone Lane - Hatch End - Carpenders Park - Bushey - Watford High Street - Watford Junction
Once District (now GWR): West Ealing - Hanwell - Southall - Hayes & Harlington - West Drayton - Langley - Slough - Windsor & Eton Central
Once District (now c2c): Leigh-on-Sea - Southend Central - Shoeburyness
Once East London (now Overground): Shadwell - Wapping - Rotherhithe - Surrey Quays - New Cross & New Cross Gate
Once Metropolitan (now Chiltern): Great Missenden - Wendover - Stoke Mandeville - Aylesbury
Once Northern (now Great Northern): Drayton Park - Essex Road

Two others up for discussion
Once Central (now Epping Ongar Railway)
North Weald - Ongar [the argument against being the limited specialty service]
Once District (now Overground)
South Acton [the argument against being that all the others still use the same tracks, this doesn't]

I don't see why the two 'up for discussion' shouldn't be included, as you only said 'stations that were once, but are no longer served by the Underground'. You didn't mention frequency or sharing tracks.