A mild spell in Autumn is a good time for planting many things. If you are struggling to choose which variety of plant, tree, shrub or even vegetable to go for, one way of dividing the good from the indifferent is to pick a variety that has earned the Royal Horticultural Society’s Award of Garden Merit. To (roughly) quote the RHS website at http://www.rhs.org.uk/Plants/Plant-trials-and-awards/Plant-awards/Award-of-Garden-Merit-(AGM):
The AGM is intended to be of practical value to the home gardener. It is awarded therefore only to a plant that meets the following criteria:

It must be of outstanding excellence for ordinary garden decoration or use
It must be available and of good constitution
it must not require highly specialist growing conditions or care or be particularly susceptible to any pest or disease
It must not be subject to an unreasonable degree of reversion in its vegetative or floral characteristics.
Though growing conditions and plant types may vary, the purpose of the award is always the same: to highlight the best plants available. The AGM is only awarded after a period of assessment, drawing upon the knowledge and experience of a wide range of members, including specialist growers and well-known horticulturists. Assessment for AGM takes place often by running trials at one of the Society's gardens or at some other venue. If you have visited one the RHS gardens you may have see a trial in progress.

Some of my favourite plants that have the AGM include:
Anenome blanda ‘White Splendour’. This is best planted after giving the little tubers a good soak in Autumn. It looks lovely in a woodland setting and will stand some shade, growing to 6” high, with delicate, almost ferny leaves . Several of my favourites Narcissus have the awards – February Gold, Jenny and Tete a Tete. All good ‘doers’ which flower year after year without a lot of tlc. Many ferns are listed. One of the easiest, even thriving in dry shade, is Dryopteris felix-mas. I have a little group by a seat under a tree and it really does look good for 10 months of the year. An unfussy shrub is Buddleia ‘Pink Delight, a great plant for butterflies. Even the humble foxglove has won the award, albeit its more cultivated cousin Digitalis ‘Excelsior’. This is a rewarding plant, easy to grow and gives height and structure to the early summer garden. Gardeners Delight and Sungold tomatoes are some of the many vegetables with the AGM. The entire list is available on the RHS website. Happy planting!