I believe that the initiating phase is the most important phase. While every phase is important in its own way if you don’t progress through the initiating phase the project will never begin. A poorly executed initiating phase will also have a huge impact on the planning phase. The entire project can get delayed if you underestimate the budget or timeline of the different tasks. Furthermore, without the customer’s approval, the planning phase cannot begin. The other 4 phases are on the same level in my opinion. You don’t want to under execute on any of them so that you can effectively progress through the project. Let’s look at the closing phase for example. If you don’t close your contracts, you’ll be continuing to pay for things that you no longer utilize. If you don’t pay workers on time, they are less likely to partner with you in the future. The closing phase is where you prove that the product is working to desirable expectations. The most important part of the closing phase to me would be logging what you learned. You get the opportunity to look back on the entire project (whether you succeed in going through all of the phases or not) and reflect on the good things that were done vs the things that could have been done better. This knowledge can then be applied to the next project.