Which word indicates a state of being extremely surprised?

The word "flabbergasted" is the best choice for indicating a state of being extremely surprised. This term conveys a strong sense of shock or astonishment, often implying that the surprise is so great it leaves one almost speechless. It captures the intensity of the emotion well, making it a vivid descriptor for overwhelming surprise.

In contrast, while "astounded" also signifies a strong reaction to surprise, it doesn’t carry the same level of exaggeration as "flabbergasted." "Unfazed" indicates a lack of surprise or being unaffected by something, which is the opposite of conveying extreme surprise. "Impressed," on the other hand, denotes a positive admiration or respect, rather than surprise.