Imagine your garden is an opera......which plants ooze charisma, and are happy to be the central character? Which plants work well to support them? Which plants are 'quieter' in nature and can be planted en masse in large groups?
Plants which take central stage for me have to have an interesting structure and year round interest. So evergreens, and small trees with attractive bark as well as forms such as pollards or weeping, or unusual 'specimen' plants are a good choice. If it is the flower which is the main feature, I would choose something which flowers for a long time, preferably a whole season.
Examples of 'Primadonna' plants*
Araucaria (Monkey Puzzle), Dicksonia antartica, Buddleja alternifolia, Gunnera manicata, Acer palmatum,Yucca rostrata, Cornus controversa, Phormium tenax, Parrotia persica, Prunus cerasifera pendula, Corylus avellana 'Contorta', Stipa gigantea, repeat-flowering cultivars of Rosa.
These are plants which have an impact, but look their best with another plant. For an example, a Tree Peony, which has striking leaves throughout the summer, and a brief flowering period of highly attractive flowers in early summer deserves a good spot in a border, but perhaps not central, because it doesn't look very good in the autumn or winter. The same thing might be said of Philadelphus (or 'Mock Orange') whose scent is intoxicating, but when not in flower, hasn't got very exciting foliage in the summer, but can work well as a backdrop for other earlier flowering plants or a centre stage grass such as Stipa gigantea. Wall shrubs and perennial climbers are often in this category.
Examples of 'Supporting Role' plants:
Philadelphus, Syringa, Buddleja davidii, Paeonia suffruticosa, Berberis thunbergii, Chimonanthus praecox, Escallonia, Cotinus, Forsythia, Hamamelis x intermedia, Cotoneaster, Garrya, Lonicera, Humulus lupulus 'Aureus', Ceanothus, Schizophragma hydrangeoides
These are plants which add texture and colour at various points and can be planted in repeated clumps to add consistency and rhythm to a border. I would include annuals and smaller ornamental grasses in this category as well as what some people call 'ground cover' or 'edging' plants traditionally planted at the front of a herbaceous border.
Examples of 'Chorus Line' plants:
Stipa tenuissima, Phlox paniculata, Geum, Argyranthemum, Alchemilla mollis, Nigella damascena, Meconopsis cambrica, Astrantia major, Scabiosa atropurpurea, Centaurea montana, achillea millefolium, Hakonechloa macra 'Aureola', Geranium, Euphorbia amygdaloides, Lavandula, Oreganum vulgare, Verbana bonariensis
*N.B. All of these plant choices are purely personal to me as a way of helping me understand how plants work together aesthetically. If you think a plant I've put in one category belongs in another, or even belongs in more than one, feel free to put me straight!