In most contexts, use who, whom, or whose to refer to persons, and use which or that to refer to animals or things. Which is reserved only for animals or things, so it is impolite to use it to refer to persons.

Although that is sometimes used to refer to persons, many readers will find such references dehumanizing. It is more polite to use a form of who—a word reserved for people.

note: Occasionally whose may be used to refer to animals and things to avoid the awkward of which construction.