Not as many i could pick here, at least not without major spoilers and, as i have friends who are just starting to watch the series, i'm trying to avoid those, we'll go with this one.
The moment Mycroft realizes exactly what he's done.
And what it's cost him. (note the cake stand next to him. i think the diet is out the window)
i haven't watched the episode, but Mark plays this so beautifully that i had to pause and watch these sections when i was flipping through Fall at camp. i have a suspicion as to all of this, but at the same time without watching it, i just don't know if i'm right or not. if they're going by cannon, then i am. if they're not, well, we'll just have to see then.