I think the most important are probably
Announcements,
Feedback, and
Member Discussion.
Announcements is a basic board for important forum news to be posted. This should be at the very top of your forum.
Feedback is a place for comments and criticism, both very important when making a successful forum.
Member Discussion is someplace for members to discuss personal matters and introduce themselves, etc.
All my forums have had these boards, and they are by far the most popular

.