Buddleja has many attractive flowering colours all ranging around mauve, pink, purple and white and well-known varieties are: B 'Black Knight' which is dark purple, 'White profusion,' 'Harlequin' pink which also has variegated leaves, and 'Empire Blue'. All Buddleja davidii are fragrant. B Buddleja alternifolia, which is also deciduous, and has lovely scented lilac coloured flowers in early summer.
A good starting point is the RHS garden merit shrubs with the RHS awarded of garden merit logo.
Buddleia davidii Nanho White 'Monite' scented compact up to 1.5m
Buddleia davidii Nanho Purple 'Monum' around 2.smaller purple flowers
Buddleia davidii 'Black Knight' scented very dark purple up to 3m.
Buddleia davidii 'White Profusion' large up to 4m and scented
Buddleia davidii 'Royal Red' large up to 4m scented red flowers
Buddleia 'Pink Delight' up to 4 m smaller pink flowers
Buddleja davidii 'Darent Valley'
For a smaller garden, B. ' Sugar Plum' raised by Peter Moore it is a red variety which is compact, growing to around 1-1.5m (3-5ft). Peter Moore has bred many varieties and Longstock park Nursery near Stockbridge in Hampshire houses the national collection, definitely worth a visit during July and August when it will be full of flower, scent and butterflies.
Buddleja provides lovely later summer colour and look well planted with other silver foliage plants such as Nepeta, Artemisia, also with tall grasses such as Stipa. There are some very compact Buddleja suitable for containers and patios such as such as B. Buzz, Blue Heaven, and B. morning mist. More compact varieties are being introduced to the market all the time, and the compact varies grow well as container plants.