I'd like to nominate Marylebone for this one. The selling off of the land around it in the 80s means any development I.e further platforms ( which are needed) too expensive. It also results in compromises eg p4,5,6 being a good walk away from the concourse
Could I also put Euston but platforms 8-11 only. The part near the ticket barriers and the ramp, which often gets dangerously congested ( especially if the LM style of " announce platform as close to departure time as possible " is followed), due to the limited circulation area near the LM Ticket office, and due to the tunnel from the underground, which is intensively used
Oxford Circus. Too small to accommodate the huge crowds now, frequently closed during peaks
I also think Baker Street is too small for its current usage, but can appreciate the historical issues involved with any potential rebuild