The one where Sutekh places his mind inside the Doctor's...
Sutekh's emerald eye rays seem extremely powerful and painful, judging by the way the Doctor writhes in abject agony. Tom Baker was always very good at playing pain and agony, he always seemed so convincing. Sutekh's powers genuinely appear to be absolute, and some of the things he says are, as Melanie Bush might say, utterly evil. He says that, if he wishes, he can keep the Doctor alive for centuries, racked by the most excruciating pain, or he may choose to shred his nervous system into a million fibres.
The confrontation between the Doctor and Sutekh is well written and performed, with some juicy dialogue for both by the hands of Robert Holmes and Lewis Greifer. Sutekh refers to the Doctor as a "plaything", and justifies his destructive tendencies quite admirably: "Your evil is my good... Where I tread I leave nothing but dust and darkness. I find that good." The subsequent abasement of the Doctor before him is quite upsetting for younger viewers. It must be one of the very few times we witness the Fourth Doctor in such a pathetic, helpless state before his adversary.