As Dr. Simon said, the DDP is often just used as a boiler plate placeholder in the DHF but it should be used as an underlying plan for the project. Therefore, all of the 5 parts you mentioned are important. If I had to pick which was the most important I would narrow it down to quality management plan and team member lists/responsibilities. Quality management is vital because it establishes the standard that needs to be met such as customer needs and also shows how the project will accomplish it. Without quality management the project could easily drift from accomplishing its main purpose. Team member lists/responsibilities is equally crucial as it helps get the project done efficiently. Team members are going to be unique, possessing different characteristics and traits. It is essential that the project manager delegates the responsibilities accordingly to each team member’s strengths. Team member responsibilities are not only important for efficiency but for holding someone accountable if something goes wrong.