Depend - I would say that it's more dependable from you, your needs, etc... There's no rule what software should "serious forum based website" use as board.
As example, phpbb team came out with extremely powerful and free phpbb3. It's big move from version2 and worth to check it. There's amazing active community behind phpbb and most likely more candies (modules/extensions) than any other pro board have.
If you're not sure what you need yet, try not to spend whole fortune for something what you maybe don't even need.