Successful container plantings require a focal point.
What makes a single plant the center of attraction?Is it color, form, height, uniqueness, or a bit of attitude?
Well, all of the above, but not necessarily at one and the same time.
A container wants a variety of plant material, either blending, contrasting or both, but the center plant is the one that will focus the attention for either good or bad.
The first consideration should probably be height as that will determine the relationship or proportion of the plantings to the container. It is important that the ratio be pleasing — that the height and mass of the plants be neither too small nor too large for the container.