From the Series 3 two-parter "Human Nature" and "The Family of Blood."
This story is where Martha really shines. I don't think that any other companion could have dealt with this situation well as she did. She has to keep an eye on human!Doctor while the two of them hide out from the Family, make sure he doesn't wander off and get into trouble (or fall in love... heh heh). But when the Doctor is finally needed, she's got to convince John Smith he's really the Doctor and not this crappy human personality as well as contend with the culture of the time period she's stuck in. And she does it brilliantly. This is one of my absolute favorite two-parters and a big reason is that Martha is so amazing in this story.
The Doctor says that he only takes the best and Martha certainly proves she deserves a spot on Team TARDIS.
Quick Note of Interest: We are now ONE WEEK away from the premiere of Series 6! Thought you ought to know :)