Liz inevitably must come in towards the bottom of the list because she also didn’t get a proper depature scene. Unlike Ace, however, we at least know the fate of the character, having returned to Cambridge University to resume her research programme. And this is a very logical and understandable departure for her character, given that she never asked to join UNIT in the first place and given that when she was virtually conscripted into UNIT, it was because, initially, they needed a scientific advisor because they didn’t know that the Doctor was going to show up on their doorstep exiled to Earth. Once the Doctor arrived, Liz’s role with unit changed from Scientific Advisor to Assistant to the Scientific Advisor, which wasn’t even what she was drafted in for. It also made her position in UNIT to be somewhat redundant since the Doctor could also handle everything (and more) that Liz could scientifically-speaking. The fact that the Doctor didn’t need a scientist to be his assistant at UNIT is commented upon in Terror of the Autons episode one, the first story following Liz’s depature from the series, as the Brigadier repeats to the Doctor what Liz apparently remarked upon a number of times. It would make sense that after a year, Liz would be allowed to return to Cambridge to resume the career she actually wanted.

The only disappointment, and the reason this comes in so low, is the aforementioned lack of a farewell scene. Some have argued that there is one in Inferno part seven because the Doctor says “Goodbye Liz - I shall miss you” before going on to insult the Brigadier. But given that the Doctor’s back about 35 seconds later and he’s the one who stays (it’s him saying goodbye to her, not the other way round) it’s tough to take that as a proper farewell scene. The fact is the audience only discovers that Liz has left when Jo shows up in the next story and tells the Doctor “I’m your new assistant”. Liz’s departure works for her character and we do get a final image of her laughing, but her depature being announced by Jo the following year means that her departure can only be so high on this list.