The Mercy of River Song vs. Scrappy Slam
It's a woman who scares even space-Nazis vs. that time Scooby Doo: Mystery Inc. slammed it's own past... hard.!
The Mercy of River Song
The case for The Mercy of River Song
Introduced as a sometime companion of the Doctor who he keeps meeting in anachronic order, River Song is deliberately enigmatic during her first few seasons on the show - partly to avoid creating a paradox with her future knowledge of the Doctor's life, mostly because it amuses her to see him flustered. The only concrete fact about her is that she's an incredibly dangerous person to be on the wrong side of, as best seen when she faces off against a Dalek. For clarity, Daleks are the show's equivalent of Nazis, terrifying even to the Doctor - they're genocidal cyborgs whose response to most lifeforms is to try to exterminate them. River, however, inspires a slightly different reaction.
The case for Scrappy Slam
This doesn't really need any introduction or explanation...