Which blue grid cell should I choose to delineate my basin?

The blue grid cell you choose to delineate a basin should not be precisely at the confluence intersection, but offset by a grid cell or even two grid cells. If you want only the incoming tributary, be sure to choose a cell or two upstream of the confluence on the tributary to avoid including the mainstem. Or, if you want to include the mainstem, but do not wish to include an incoming tributary, choose a grid cell upstream of that confluence on the mainstem. Finally, if you wish to include the tributary and the mainstem in your basin, it is best to choose a cell or two downstream of the confluence to ensure that both are included.