What always impressed me about Nancy Cartwright is the range of character types she's voiced. While it's pretty obvious that Bart Simpson and Bright Eyes (Pound Puppies) have the same voice if you listen to them, it's just funny to think how different those characters are. (Though I suppose that's more about the writing than the voice, but I hope I make some kind of point here)
I always liked Rufus from Kim Possible, and I think that's mainly because of Nancy's voice. She has voiced quite a few characters I remember fondly.