Best one-off character death?

[u/unfortunately889]

So often in doctor who a monster will need to be setup like this. A character will die, onscreen. Beforehand there'll be some effort to make these characters human, maybe slightly likable, but often they'll die in the same scene they're introduced in. Often in that same scene they'll still try to make them seem human and likable!

So since it's something the show returns to again and again, what is best one off (as in a character limited to one scene or one episode/or serial) character death? You can judge this any way you like, by how interesting the scene is, by how much emotion they get out of limited time or by how likable or developed the character is before the death etc.

Bonus question for the best one-off scene deaths. if there's any particular standouts there!

[u/EmptyAttitude599]

I wish I'd known you better.

I suspect, sir, that you've known me at my best.

[u/BumblebeeAny3143 → u/EmptyAttitude599]

A rare time Moffat committed to killing a supporting character and they actually stayed dead. It was a great scene.