Most have been mentioned here already, but as I'm a newbie, I'll kick in a few of my own. I go back a VERY long way, and could cover many Scottish branch lines which got a Beeching's powder in the 1964/67 decimation, but that might be a bit boring, just to list loads of closed lines that I used from time to time - but the one that I used frequently as I used the train to/from town a lot was
Barleith, Galston, Newmilns, Darvel.
Also in Scotland, Hawick, Galashiels, as I had a weekend trip with overnight stops just before the Waverley Route closed.
More recently, on the Southern,
all stations on the Winchester - Alton route
Woodside, Addiscombe, Bingham Road, Coombe Road, Selsdon
Coulsdon North
Weymouth Quay
Waddon Marsh, and stations to Wimbledon
Bridport
And on LT
Ongar - and another "last" not railway related there was that there was a pub near the station, called The Cock, and I had my last pint of Grays beer there also.
Aldwych
Hounslow West terminus
East London Line inc Shoreditch
As an aside, and a deviation from the thread, I can also claim to have visited while closed and now re-opened, Dalmarnock, Bridgeton, Glasgow Cross, and Glasgow Central Low Level as there was fairly easy and virtually unimpeded access to the tunnels, provided one wore waders, and was prepared to navigate a load of debris and three feet of floodwater at the tunnel mouth.