For a flying wing airfoil, which AOA is to consider when selecting Cl?
Do not look at the angle of attack. Instead, focus on ratios like best lift-to-drag ratio. The angle of attack will be a consequence and not a design parameter.

First you need to determine what your flying wing will be good at. Next, determine which propulsion (jet or propeller) it will use. Then you can work on the layout (aspect ratio, wing sweep) and when that is decided, only then you are able to determine the optimum lift coefficient.

The process to determine the optimum lift coefficient is discussed here, here and here.