Formal:
* Hola, Emily. (This is the most common and neutral way to greet someone.)
* Buenos días, Emily. (Good morning, Emily. This is appropriate in the morning.)
* Buenas tardes, Emily. (Good afternoon/good evening, Emily. This is appropriate in the afternoon or evening.)
Informal:
* Hola, Emi. (This is a more casual way to greet Emily, using a nickname.)
* ¡Hola Emily! (This is a friendly and enthusiastic greeting.)
The best way to say "Hi Emily" in Spanish depends on your relationship with Emily and the context of the situation.