Vengeance on Varos (Sixth Doctor).
I'd heard that this one was considered controversial at the time it aired but I'm not sure what the fuss was about. Sure, the Doctor made a flippant remark after a couple of guys fell in an acid vat but then again the Fifth Doctor shot one of the cybermen multiple times at close range.
I enjoyed the story, though Jondar and Areta became superfluous in part 2. Sil was a rat bastard and fun to watch.