Planning and Execution

I have been doing a lot of planning lately and I have got to thinking about what is more important planning or execution. Not to say that they are both not important. But would you rather have a great plan and and average execution or an average plan and great execution? I tend to think that you need a great plan from the start and if your execution is subpar then at least you are working productively athough not very well. If you have a shitty plan and execute it perfectly, then you really haven't accomplished that much. I think that a good plan is essential for success, but only true success can come from a well executed great plan.