Formal:
* Merveilleux (masculine) / Merveilleuse (feminine) - This is the most common and direct translation of "marvelous."
* Extraordinaire - This means "extraordinary" and can also be used to convey "marvelous."
Informal:
* Génial - This means "brilliant" or "awesome" and is a very common way to express something is marvelous.
* Superbe - This means "superb" and implies something is very good.
* Fantastique - This means "fantastic" and conveys something is amazing or wonderful.
* Formidable - This means "formidable" and is another way to express something is impressive.
Other options:
* Magnifique - This means "magnificent" and implies something is grand or beautiful.
* Excellent - This means "excellent" and conveys something is very good.
The best choice will depend on the specific situation and the desired tone.