It depends on your criteria, but if you're looking for a well-supported, vBulletin-ish forum (focus on being a forum first-and-foremost, similar visual style, similar extensibility system) then XenForo is a very good candidate. If I didn't have a lot of time invested in my vBulletin setup (customization), I would go with them. For now, vBulletin works for me.