Answer by Professional Japanese Teacher
For complimenting her appearance
For complimenting her personality and character
XXはすばらしい人（ひと）だ。XX wa subarashii hitoda.
君（きみ）はすごいよ。Kimi wa sugoiyo.
君（きみ）はすごいね。Kimi wa sugoine.
There are several ways of complimenting people.
Suteki can be used for personality too, but she would be very happy especially if you use it to compliment her appearance. Adding final particles like “yo” or “ne” conveys your admiration.
Subarashii and sugoi are often used to compliment a person’s personality and character. “Kimi” means “you” in literature. It is often used in song lyrics but not in daily life. However, when you use in compliments, it gives off a romantic vibe. Try using it when sending messages to your girlfriend.