As with Rose, there are many excellent Martha moments to choose from! I went back and forth picking this one or that one but I kept coming back to one particular moment so that must be the one. :D
Martha Laughs at the Master in Last of the Time Lords
Now I have issues with this episode but none of my issues have anything to do with Martha. She is in fact about the only reason that I would even consider watching the first half of it ever again. (No, John Simm isn't enough. I'm sorry but it's true.) Martha, on the other hand, is made of awesome here; she walks the Earth and then laughs at the Master when he's right in the middle of doing his best evil overlord gloat. One laugh from her and he's wondering what the hell is going on shortly followed by all of his evil plans being overturned. Ha.