Which element of projects you believe is most difficult to manage?

The Unknown - Unknowns risks, as an example Sept.11,2001 World Trade Center attack in New York city

I think it depends on your role in the project, for example if your a client your difficulties will differ than contractor. So basically you have to see your own difficulties. common ones are to meet the deadline with quality and within the budget and in a safe work environment.

I think the most difficult to manage when you're working with a team are: lack of cooperation among members, and; having incompetent team members.

Unexpected risks and Project organizing phase  

Well, It Basically Depends upon projects and clients.But as far as my experience said, its the assessment of of risks and deadline for meeting the objectives.

Customer Satisfaction with the achieve of target dates to completion the project.

Stakeholders

I think it depends on the kind of the project, but generally i think it is HR.

Tasks,Schedule,Critical Path

Risk