The one where the Drashigs break out of the miniscope...
I love the look of disgust on Shirna's face when the Doctor emerges from the miniscope and begins to grow. Everybody seems to view the Tellurians (ie, Earth people) as a lesser species, despite the fact they look just like Lurmans. Shirna shows the disorientated Doctor some compassion when he's coming round, like a nanny might a child!
I also love how the Doctor completely takes over the situation the moment he arrives, questioning the Inter Minorian tribunal, reprimanding the Lurmans for operating the scope illegally, and blaming Pletrac for allowing its importation. Pertwee is on fire in this scene, commanding the set and putting every character in their place.