Depends whether you want something primarily for foliage or flowers. If it's foliage, I would recommend a Paulownia, which has extremely large velvety leaves and after a mild winter, beautiful 'foxglove' like flowers. Although it will grow to a very large size, it can be cut back every year which means it will produce even larger 'tropical' size leaves (but you lose the chance of the flowers). It is very vigorous and quick growing.
If you just want a shrub, which is what 12 ft says to me, I would go for one of the honeysuckle/lonicera shrubs such as Lonicera fragrantissima which has beautifully scented flowers in winter. It also grows very rapidly and is dead easy to take cuttings from.

Thanks for that.

Just bear in mind that anything which grows quickly to 15 feet is likely to carry on growing quickly to 60 feet.
A smallish tree for a not too cold wet site would be Halesia carolina, the Snowdrop tree.

How about a Salix that is grown for the coloured winter twigs? You pollard it every year so it never gets large. Try Salix alba britzensis.

Crataegus monogyna the common hawthorn will do very well in a moist situation. Lots of flowers in Maytime, white, rose and scarlet with double and single flowers. Good for bees and birds with haws for winter interest.