Margaret Meloni asked (over at IT Toolbox) the following question:

Take one Body of Knowledge from the Project Management Body of Knowledge and defend it as the most important. 

My answer is: They all are equally important.
 
In the early days of The PM Podcast I asked every guest the same question: "What is more important in a PM: PM expertise or industry expertise?" As you can imagine, pretty much everyone had a different opinion about that. But in the end, I came to the conclusion that you need a good mix of both in order to be a good PM.

It's the same with your question about which of the Knowledge Areas is the most important one - you need to apply them according to the needs of your project. I agree that communications is incredibly important on any project and needs special attention. But try telling to an aerospace project manager that risk is not equally important. Or explain to the PM of the 2012 Olympic Games that schedule is not something they need to worry about too much.

You need a mix on your project. You need the right mix.

But allow me to end these musings with a fun thought that takes this to a totally different level: The one that you ignored and made your project fail would have been the most important one...