The Neighborhood Trolley vs. The Mercy of River Song
It's your only happy memories of public transportation vs. a woman who scares even space-Nazis!
The Neighborhood Trolley
The Mercy of River Song
The case for The Neighborhood Trolley
Mr. Roger's Neighborhood was a warm and welcoming space where you could make any day a special day just by being yourself. A place where you were reminded daily that there was no one else in the world like you, and that people could like you just the way you are.
The Neighborhood of Make Believe was an interesting (and sometimes scary- I'm looking at you, Lady Elaine Fairchilde) world to visit, but not before Mr. Rogers eased you into the story with a comforting intro and reminding you that this part was only make believe, not real life.
And there was only one way to get to either place you so desperately wanted to visit: the Neighborhood Trolley.
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.