I think the reason we all find it difficult to name a favourite plant is because they are all favourites for different reasons whereas with plants we hate we generally hate them for the same reason.Plants that herald a new season like snowdrops,tulips, roses, callicarpa, holly give us something to look forward to.Scented plants like freesia, honeysuckle, lavender, lilies often come with memories.Other plants we love just because they survive and grow for us no matter what we or the weather throw at them - daffodils, peonies, hardy geraniums and trees that shower us in blossom, shade and leaves year in year out.Fashion and taste also affect our choice - dahlias, gladioli, chrysanthemum, pinks have all ridden the rollercoaster.
At the moment my favourite plant would be Fremontredendron 'California Glory'as I look forward to seeing it's swelling buds before it bursts into flower in May and stays in flower until November covering my neighbour's garage wall beautifully.