I would have to agree that out of the five listed, effective communication is key for any project. From my experience, alot of projects have teams that are spread across evenly throughout the world. Effective communication is key to allow everyone to be alignment on key deliverables as well as important changes to the projects. Having everything else listed is important but there could be numerous iterations or changes to a project throughout its course that can affect scope, plan, and objectives. Proper communication would allow all these changes to be understood and communicated to key stakeholders.