Its a very tough question as, the answer is dependent on the type of project. However from an overarching view, all tasks are very important as they are all interdependent and connected to each other. Similarly their isn't a phase that is "least" important like initial phase as the user needs to fuel the project and initiates the idea on what user is asking for, if this isn't clear and defined the whole project/product will not have any substance. 

I believe all subsections of the project life cycle are crucial for the ultimate success of the project. However, the planning phase and quality control throughout the whole cycle could be the most critical for the medical device industry. If the planning is very detailed with multiple backup plans then the execution phase becomes much easier. And for ultimate success and to get FDA approval for devices, quality assurance and control are essential. Any minor imperfection in quality control could lead to FDA rejection. Therefore, the workers should provide the most sincere attention to quality assurance.

In my opinion, all parts are essential in the lifecycle because they all are necessary. After all, you can’t complete the cycle with more of one step and less of another, the cycle would be incomplete. However, with that said the planning stage seems to be the most crucial step in medical devices, because it has to be thought out thoroughly and one would also have to think of alternative options, and have a plan a,b, and c, along with taking into account any what ifs or mishaps. Planning is essential and crucial in many aspects of life. For instance, if you are building something unique or elaborate, it would be quite difficult to just start the process, whereas if you have a plan, you can always revert back to it and tweak it if necessary. If you hired a contractor to build your family home, would you trust him to do so without a plan? Would he be efficient at working from memory?  

The initiating phase is the most important phase of the project life cycle. As this is a medical device course forum, it feels slightly appropos to use a biological analogy. In order for a cell to replicate in mitosis, it must spend most of its time in interphase to replicate its genetic content and prepare for replication before actually replicating. Similarly, preparation is highly important for teams to proceed. Although errors are inevitable, the initiating phase can help avoid errors later in the project. The initating phase is the stepping stone for due diligence to be fulfilled later in the project. Adequate planning is essential, but can only follow the necessary preparation of the initiating phase. In the initiating phase, the grounds for communication can be established, with clear expectations from the customer. Because the customer is the most important stakeholder, the deliverables defined in the initiating phase help establish the factors that will keep the customer happy.