The cocooning scene. The effects were astonishing at the time (though they are a bit dated), and Michelle Williams does an excellent job at conveying the agony and terror young Sil feels as her body moves forward with its life cycle without her consent or understanding.
Sil faking her death is quite an impressive plan for someone with only a few days experience in the real world. Special note to getting four huge open containers of gasoline to ensure the car does explode when she crashes it, somehow having already learned Every Car Is a Pinto is a trope, not a fact. Then, with a little hair dye, she is able to walk up to and even talk to the people who had been hunting her, since only Dan saw her face at the club when she lured them into the chase.
The overall performance of Natasha Henstridge. A model with zero acting experience (but eager to get into acting so she could get out of modeling), she gives Sil a surprisingly level of complexity and empathy when she could have easily been a one-note monster. Her inexperience perhaps sometimes even worked in her favor, as Sil would logically be just slightly off from everyone else, due to the combinations of her short life between birth and physical maturity, her being raised in isolation since her escape, and her alien nature and instincts operating in her brain.
