Formal:
* Mauvais odorant: This is the most literal and formal translation, meaning "bad smelling."
* Nauséabond: This translates to "nauseating" and is used for particularly strong or unpleasant smells.
Informal:
* Puant: This is the most common and informal word for "smelly."
* Fide: This is a more slang term, similar to "stinky" in English.
* Sentant mauvais: This literally means "smelling bad" and is a more casual option.
The best word to use will depend on the situation. If you are talking about something that smells bad, like a piece of garbage, "puant" or "sentant mauvais" would be appropriate. If you are describing someone who smells bad, "fide" or "mauvais odorant" might be more appropriate.