Formal:
* Bienvenue: This is the most common and formal way to say "welcome." It's appropriate for any situation where you're addressing someone you don't know well, or in a formal setting.
Informal:
* Bienvenue: This is also used informally, but often with an added "à" (to) before the person's name. For example, "Bienvenue à Marie." (Welcome to Marie)
* Salut! This is a more casual greeting that translates to "hello," but can be used as a way of welcoming someone.
* Bienvenue chez nous! (Welcome to our home) This is a more welcoming phrase used when someone is entering your home.
In response to someone welcoming you:
* Merci! (Thank you!)
* Merci beaucoup! (Thank you very much!)
The best way to say "welcome" in French depends on the context and the level of formality you want to achieve.