If you want to plant just one slow-growing, small tree the two best families to explore are the crab apples and the hawthorns. They offer self-fertile spring flower and autumn fruit. They are undemanding when it comes to soil and situation and they are completely hardy - even in exposed situations.
I would recommend the following crab apples for their beauty and robust health.
Malus x robusta 'Red Sentinel' is a green-leaved variety with white flowers and clusters of deep-red fruits and these remain on the tree throughout winter.
'Evereste' is a small, conical tree with white flowers, softened by pink-red buds, and the large orange-yellow fruits are about an inch wide (2.5 cm).
'Rudolph', a small upright tree, has darker bronze-red leaves, large rose-red flowers and small orange-yellow fruits.
'Montreal Beauty' is a green-leaved crab apple with large white flowers followed by red and yellow fruit.
The hawthorns are unusual because they can survive salt laden winds and polluted air. Some (like Crataegus crus-galli) have long and vicious thorns but the following are much less savage. Crataegus orientalis (my own favourite) has cream flowers studded with red anthers followed by warm-orange haws. The soft, grey-green leaves are finely cut and this small, airy tree can be used above silver foliage.
A more traditional hawthorn with double, pink flowers is C. laevigata 'Rosea Flore Pleno'. I find this softer on the eye than the brighter pink 'Paul's Scarlet'. But both only form a few haws so if it's an abundance of deep-red fruit you're after opt for Crataegus prunifolia. This compact, but wide-headed tree, has white flowers with red anthers and the branches stretch out almost horizontally. In autumn the vibrant orange leaves are quick to fall and they reveal a heavy crop of bright-red, cherry-sized fruits on bare branches. The fruits gradually scatter on the ground creating a late spectacle in the garden.
Bare-root or containerised?
December is an excellent month to plant bare-root trees as long as the soil is frost-free but I wouldn't plant anything on heavy clay soil until after late January. Bare-root trees are planted when dormant, between the months of November and early March, and there are definite advantages. The trees are generally cheaper and, although smaller, they race away often outstripping larger, more-expensive container-grown specimens.
Container-grown trees can be planted throughout the year, although never in extreme heat or cold. Always avoid trees that are potbound, that's when the roots go round and round within the pot, and always ask to see the roots when buying. Use a local specialist tree nursery whenever possible. They will give you excellent advice (for your area) and their greater turnover will mean that your tree is likely to be in tip top condition. Too often trees in garden centres are kept in the same pot year after year and never recover.
How to plant an ornamental tree
Buy the tree stake and tie when you buy the tree and you can also buy circular mulch mats to keep the weeds down. This helps a new tree enormously.
Select an open site where you can appreciate your tree. Don't tuck it away on the boundary edge.
Dig a large hole and add organic matter, whether it's garden compost or a soil-based compost, and then plant your tree carefully. Match the height of the soil in the pot to the soil surface because many trees are grafted. This graft needs to be above the ground to avoid suckering.
Carefully stake and secure the tree straight after planting, leaving it in place for three to five years, but do loosen the tie every few months.