The Face of Evil (Fourth Doctor).
A good introduction for the character Leela. A computer gone haywire story where the computer isn't destroyed is a new one in my experience. I also wish they had devoted more time to the backstory of Xoanon. Then again I think it would have been cool to actually film the backstory as a seperate serial and then have the doctor come back several centuries later to find out the results like he did in The Ark
back in the Hartnell era.