I believe the most important stage is monitoring and controlling. While initiation is important, components such as time and budget can be adjusted if necessary. In regards to planning, while it is very important to plan out the project, it is just as common for delays and other things problems that come up that result in the plans changing. For this very reason, I believe that monitoring and controlling is the most important because the manager has to continuously check that status of the project and ensure that it is on track. If it is not on track, then it is their responsibilities to make adjustments. 

It's difficult to determine the relative importance of the steps of the project lifecycle because they are all necessary to complete a project. It's similar to looking at a perfectly functional four-legged chair and asking yourself "Which leg is the least important?" Each stage of the process has its place and need to be present in every single project.

Although, I do have an opinion on the question "which step in the project management life cycle is the most important to perform effectively?" In my opinion, it is absolutely vital that the planning phase is done well. The planning phase lays out the groundwork for everything in the project: the schedule, the people involved, the tasks, the scope, the finances, etc. There's a correlation between the success of a project and how well it was planned. Poorly planned projects are doomed to behave wildly, with unexpected problems occuring for many tasks. Again, all steps are important to do well, it is just my belief that planning is the most important in this regard.

While all steps of the project management cycle are necessary and important, I believe that planning is the foundation for success in a project. I often hear this saying (I'm not sure where it's from), "By failing to prepare, you are preparing to fail." Having a well-thought-out and organized plan is the foundation of a project and will make it easier to execute and close out the project. Plans can be adjusted as problems arise during execution, but that does not negate the importance of a plan. How do you know that your plan is well-defined and organized? What criteria should your plan meet?

Understanding the project management phases, every phase is necessary for the project to be complete thoroughly with a successful final project. That being said, I feel that the phase that would be the most important would be the phase where the tone of the project is set. This includes defining key objectives, forming a scope, and determining the aims of the project. This phase is the initiation phase. With major flaws in this phase or items left undetermined, a project may have severe difficulty in being completed. As room for improvement is beneficial, an abundance of unknown will leave the project scope in constant flux which does not allow the project to be effectively worked on. Furthering this idea, any key objectives that are able to be changed causes the aim to and scope to be changed throughout the project.

In short, the better the initiation phase is, the easier the rest of the project should be. While there are outliers, a strong and thought-out initiation phase will allow for a thorough planning phase and simplifying the execution phase.

I really think that the starting phase is the most important, either if it is initiation where you talk about desired final product or the first few executing and feedback because if they get your project's "style" and "vibe", it will go so much more smoothly. So it's more of a strict monitoring and trying to convey the "vibe" you want at the beginning. By the time closure of a project approached, I think it's no longer a good time to fix things (even though it's the "debugging" time basically)

I believe the planning phase is the most crucial step in ensuring a successful closing. Initiation defines the need and objectives, and execution brings the work to life. Planning provides the structure that connects the two. A strong plan solidifies the scope, defines timelines and risks, organizes tasks, and estimates resources, giving the team a clear roadmap to follow. When planning is done thoroughly, closing becomes smoother because expectations were clear from the beginning, deliverables align with the set objectives, and risks have already been addressed. Without effective planning, even the best execution can result in missed goals, delays, or incomplete closure.

I once worked on a team that failed to develop a detailed plan before moving into the execution phase. Our timeline only highlighted major deadlines and did not break them down into smaller tasks. As a result, we faced repeated delays throughout the project, which ultimately prevented us from addressing a major objective.

As others have said, I also think planning is holds the most weight in ensuring a project close out. As others have already said, the planning phase provides the foundation for which the project execution is based on. Good planning makes the whole project run smoother, as identifying tasks well in advance of when they need to be executed is absolutely critical for avoiding unnecessary delays. How many times have you had to execute a task at work or even at home, just to go grab the materials or tools that you needed only to find them missing, or in use by someone else, or expired, broken, or just not being exactly what you needed? While at home this is usually only a minor inconvenience, it may result in serious delays if it turns out the equipment you need is actually undergoing routine maintenance that was forgotten about during the planning phase. Good planning can account for minor details to prevent unnecessary delays and keep the project running smoothly. However good planning must also be flexible enough to account for even unforeseeable delays. An overly rigid plan may be able to come up with a theoretical fast deadline which sounds good on paper, however, this plan will also suffer from even minor unexpected delays cascading into subsequent tasks causing larger and larger delays and problems. While monitoring and controlling would be important here, good flexible planning will reduce the burden on monitoring and controlling, by building leeway into your tasks, and preventing non-critical paths from suddenly becoming the critical path due to bad luck. I believe that more hours put into planning pays off far more than other phases because for every extra hour of planning you can save far more than that time spent, simply by accounting for problems before they happen.