Final answer:
The apical four chamber view is the best 2D echo view for assessing the interatrial septum.
Step-by-step explanation:
The best 2D echo view for the assessment of the interatrial septum is apical four chamber. In this view, the ultrasound probe is placed on the chest in the fourth intercostal space along the left sternal border. This allows visualization of both atria and the interatrial septum in a longitudinal plane.