This is the 5827th most frequent Japanese word.
ひときわ
"ひときわ" means "remarkably" or "conspicuously."
In this sentence, "ひときわ" is used to emphasize the exceptional brightness of a star in comparison to others.
ひときわ明るい星が輝いている。
A distinctly bright star is shining.
Here, "ひときわ" highlights how her clothing attracts attention apart from other surroundings.
彼女の服はひときわ目立っている。
Her outfit stands out distinctly.
In this case, "ひときわ" is applied to stress the notable height of a building compared to others in the area.