I believe that all the steps are necessary for a successful project but the most important phase within a project life cycle is the planning phase. The planning phases is where you gather all your information on the project, make a schedule, create a Gantt chart, figure out the costs, look for resources, create teams, initate meetings, and solidifying your goal for the project. All these steps are very important for planning a project and making all the steps move more smoothly. If these steps are not thought out carefully then further down the cycle will become more difficult to create a successful project. From my experience in college, whenever we were working on a project and it was not planned properly, there were always set backs later down the road which would hold up the project.
The least important phase in my opinion would be the executing phase. This phase deals with having regular meetings and keeping up with the plan but this can also be done in the monitoring and controlling phase. The monitoring and controlling phase is where you make sure all everything stays on track, keeping up with the budget, utilizing your resources and doing all the calculations. The meetings done during the executing phase can also be done during the monitoring and controlling phase and the schedule is already made during the planning phase.