Jackie - While I do believe the business view is ultimately the most important you make a very good point. I think the the business and architect's view somewhat go hand in hand. All of the layers go hand in hand, however I think these two must both be as accurate as possible to ensure that the layers following them are successful. I believe that the business model is the most important because everything is molded around the requirements given in this initial layer, however the architect's view seems like is would be the most difficult to get correct. In my opinion gives this layer a strong case for the most important layer.

I strongly believe that the designer's view, AKA the logical security architecture, is the most import layer in the SABSA model. The role of the designer is to take the architect's vision and translate it into reality. The power inherent in this role could make or break the project. This role also requires great skill, as the designer most possess the ability to understand the concepts of architecture and engineering to bring the beginnings of a plan into being. The designer is essential for laying the foundation which turns the abstract into form.

This almost seems like a trick question. Everyone is giving great arguments as to why each layer is the most important, it's almost difficult to stick to my original post after reading all of these different comments. A common theme that everyone seems to be agreeing on however, is that almost all of us selected the 2-3 upper layers of the model as the most important.

I think the Business View is the most important layer in the SABSA model because it lays the foundation of how everything will be constructed. All of the layers work hand in hand with each other but like a building a building, it needs a foundation to give it stability so your security plan has to have stability so the business view gives stability to your security plan.

Jackie, I see what you're saying about the architect view, but if the idea (or in this case the building) never gets to the architect, then there is nothing to invision.  So you need the business view to get the ball to the architect's court, wouldn't you agree?

Doing a little additional research online I found a different model of the SABSA model in which the "operational security architecture" is placed vertically through the first six steps.  SABSA's explanation for this is that Operational Security arises (or has "meaning" as SABSA states) through each part of the six steps.  Originally I was hard pressed to pick one specific layer as "most important" but if Operational Security has a hand in every single layer of the model than I would have to say that this is the single most important part of the six layers.  After all, if you get that layer wrong, the rest will be wrong as well.