I think all phases of project management are very crucial for the project. Each project has to go through all 5 phases successfully to have an ideal outcome. If I was to rate the phases based on the importance and risk factor, I think project planning and execution would the most important phase. All phases are important but planning and execution phases are where all the resources are allocated, the project is being implemented, and all the risks, resources and quality are managed.
Execution phase is completely dependent on the planning phase. Project Execution is where you perform the assigned tasks and the work and efforts are performed to meet the outcome of the project. Project Planning is where you manage the project plan considering resources, budget, schedule, quality, communication and risk.