Formal:
* Chica hermosa: This is the most literal translation and is appropriate for most situations.
* Muchacha hermosa: This is also a formal option, but it emphasizes youthfulness.
Informal:
* Niña bonita: This is the most common way to say "beautiful girl" in a casual setting.
* Bella: This is a more general term for "beautiful" and can be used for both girls and women.
Other options:
* Hermosa joven: This is a good option if you want to emphasize that the girl is young and beautiful.
* Guapísima: This is a very strong and affectionate way to say "beautiful."
The best way to say "beautiful girl" in Spanish will depend on the context of your situation.