"Struck" simply means it caught your attention. It might not involve any strong emotional reaction. "I was struck by how easily I could see flashes of red as the red-winged blackbird flew." "Surprised" emphasizes the fact that you didn't expect it. You might be surprised in a good or a bad way, and you might experience a strong or a mild reaction. "I was surprised to find that a Hershey's Gold bar doesn't have any chocolate in it." "Shocked" means surprised in a unpleasant way, and experiencing a strong negative reaction. "The phone rang. I was shocked when Sandra told me that Roy had died. He had seemed so strong and vigorous the day before." "Taken aback" is very similar to "shocked." Literally, it is a nautical term. It happens when a ship tries to tack, does not complete the maneuver successfully, and gets stuck in a situation in which the wind is driving it backwards. "Taken aback" carries the additional idea of being paralyzed with shock, frozen with dismay, and having an empty feeling of just not knowing what to do. "When Sandra told me that Roy had died, I was completely taken aback. I was literally speechless for a minute. The words to express my condolence would not come."

Struck is usually struck by something. It's to become aware of something in particular very suddenly. I was struck by her beauty. I was shocked and I was taken aback both work. You can add "very" or "completely" in front for emphasis in this situation.