You get to exchange one existing serial for one missing serial. Which do you trade?
I'd give up the Gunfighters because the story is just.. kind of lame, really, and exchange it for The Daleks' Master Plan, it's a great serial filled with action and sets which would benefit from being on video.
The Daleks' Masterplan is an obvious candidate for a missing story, but I think I'd also be happy to swap Planet of Giants for Marco Polo.
Alternatively, I'd swap the two surviving episodes of The Crusade for the missing two episodes of The Reign of Terror. The Crusade doesn't really work for me, as it is unlike most other 60s historicals where there is a clear historical arc (e.g. buildup to the burning of Rome in The Romans or the fall of Robespierre in The Reign of Terror).