Formal:
* Vous êtes magnifique. (This is the most formal and polite way to say it.)
Informal:
* Tu es magnifique. (This is more casual and used with friends or family.)
* T'es magnifique. (This is a very informal and familiar way to say it.)
Literary/poetic:
* Vous êtes splendide. (This is more elegant and emphasizes beauty.)
* Vous êtes admirable. (This focuses on the qualities that make you magnificent.)
The best way to say it depends on your relationship with the person you're talking to and the tone you want to convey.