Sarcococca hookeriana var digyna have the tiniest most indistinct flowers with a glorious, intoxicating scent. Or try Sarcococca ruscifolia 'Dragons Gate' which is more delicate in shape. I have them either side of the front door and along the garden path. They tuck in well among showier plants and make good ground cover. Erica- the heathers- work well too if you like them. Try Erica carnea ‘Springwood White’.
If you have space then Witchhazel- Hamamelis is super scented with leaves that blaze with Autumn colour. Same for Chimonathus and Stachyrus- both scented Spring flowering shrubs. Viburnum x bodnantense 'Dawn' and some of the other varieties have scent and flowers as do the Daphnes. Coronilla Valentina is lovely, again it is a shrub. Abeliophyllum distichum is pretty with white flowers like Forsythia.
Trollius, Aconites and Winter Iris (Iris unguicularis var. angustifolia) are all bulbs that survive deep snow - the variety "Walter Butt" is one of the best – a clear, silver striated blue. Same with Chiondoxa and Scilla. Frittilaria are another bulb that come in the Spring. Hepatica transsilvanica Blue Eyes flowers early in spring/late winter. Gentiana verna alba flowers late winter and makes a good Alpine plant for ground cover. Viola tricolor - Johnny Jump Up is another good bedding plant for underplanting.
Foliage wise, try the grass Acorus gramineus - Sweet Flag. In the winter, it has beautiful texture and green, white and chartreuse colors. Ophiopogon planiscapus has black strap like leaves and looks amazing in the snow and frost.
I have Cyclamen Coum, Hederifolium and Neapolitaneum although they flower better late Autumn and early Winter. Bergenia 'Bach' is a lovely winter perennial. Or try the variety 'Eroica'.
Clematis napaulensis has large clusters of flowers in early winter- pendulous, scented greeny-yellow bells with attractive red-purple stamens.
Tree wise, Prunus subhirtella 'Autumnalis' has white flowers throughout winter from November till early Easter time and the Japanese Apricot, Prunus mume 'Beni-Shidare' has red pink heavily scented late winter blooms but needs shelter.
The winter flowering honeysuckle, Lonicera fragrantissima and Lonicera x purpusii 'Winter Beauty are semi evergreen, scramble happily along a wall and are heavily scented.
The Hellebores work hard for their money- try the white 'Niger' underplanted with tiny violets (species) and the 'Orientalis' in vars, different shades of pinks or purples and reds.