I've always felt Charla Sedacca had the most deceptive muscles. When she was relaxed, she appeared to be a regular, somewhat thin, woman. But when she flexed, WOW. I made 2 screencaps from one of her videos a couple of years ago (forgive the poor quality) to demonstrate the difference. At left, her right arm is that of a normal woman. The photo at right shows her massive and striated biceps and delts.
I love that kind of situation, especially on the "smaller" women - I like to call it "hidden muscle". One particular moment is when they practice the "Relaxed" pose and make their lats flare.
Vanessa Serros is a great example of this.
A lot of women posted images of their muscles flexed all the time, but some of the images I love most are where you can see a clear distinction between their flexed and unflexed state:
Alcione Barreto's legs:
...taken from this vid.
Kristina Zafirova's arm:
...taken from this vid.