Formal:
* Usted se ve hermosa/o. (This is the most formal way to say it and should be used when addressing someone with respect, like an elder or someone in a position of authority.)
Informal:
* Tú te ves hermosa/o. (This is the most common way to say it to friends and family.)
* Estás hermosa/o. (This is a more colloquial and affectionate way to say it.)
More poetic:
* Eres hermosa/o como un/a ángel. (You are beautiful like an angel.)
Remember to use hermosa for a female and hermoso for a male.