Unless you'll be trying to create a super duper complete converter, the obvious are what you need

Members, categories, boards, topics, messages.
Then, still doable, polls, poll_options, log_polls, personal messages
From here on, it heavily depends on which data you have available and how much effort you're willing to invest. Attachments are possible, avatars, smiles, etc, depends. Member groups, permissions, various settings are usually left for manual tuning