Formal:
* Monsieur: This is the most formal way to address a man, similar to "Sir" in English.
* Homme: This means "man" and can be used in formal contexts.
Informal:
* Mec: This is the most common informal word for "guy," equivalent to "dude" in English.
* Type: This means "type" and can be used to refer to a particular kind of guy, like "that type of guy."
* Bonhomme: This means "good fellow" and can be used affectionately.
Other:
* Garçon: This means "boy" but can be used informally for a young man, similar to "kid" in English.
* Fils: This means "son" and can sometimes be used to refer to a young man.
The best word to use depends on the context and the level of formality. If you're not sure, it's always best to err on the side of formality and use "Monsieur" or "Homme."