I think the most important phase is planning phase since a plan can decide many things. Good plan can save time, save budget and also save energy. If you have good plan, your project is more likely to success. Also the plan should be flexible, because it is inevitable to change the plan in different situation.
I think all phases are important, but if I have to choose the least important one, I will choose the closing phase. I think the functions of closing phase is to make a summing up of the experience and lessons. I feel that it serves for future project not the current one.