Everyone. See question June 18th. That said, you specifically need to know the people who hire you. That differs from medium and budget range.
Studio Films: By the time you're being hired for studio films, you'll already know the people you need to know.
Indie Films: Independent Producers, or anyone who will give you money to direct your own film.
Commercials: Commercial Production companies. Many big ones have in-house directors. Advertising Agency Execs.
Television: Executive Producers, Network executives, UPMs, Line Producers
And of course, get yourself some director mentors!
For more tips and articles by top entertainment industry coach, The Greenlight Coach, visit www.theGreenlightCoachBlog.com