Common lilac (Syringa vulgaris) and its varieties/cultivars win for best scent, as they are extremely fragrant. There are many cultivars--2,000 at the Arnold Arboretum.  Here is the arboretum's list of lilacs with the highest degree of fragrance:

Syringa vulgaris ‘Laurentian’ (blue)

Syringa vulgaris ‘President Lincoln’ (blue)

Syringa vulgaris ‘Adelaide Dunbar’ (purple)

Syringa vulgaris ‘President Roosevelt’ (purple)

Syringa vulgaris ‘Charles Joly’ (magenta)

Syringa vulgaris ‘Ruhm Von Horstenstein’ (magenta)

Syringa vulgaris ‘Catinat’ (pink)

Syringa vulgaris  ‘General Sherman’ (pink)

Syringa vulgaris ‘Katherine Havemeyer’ (pink)

Syringa vulgaris ‘Vauban’ (pink)

Syringa vulgaris ‘Asessippi’ (lilac)

Syringa vulgaris ‘Excel’ (lilac)

Culture: Best soil is one close to neutral and supplemented with added leaf mold; faded or old flowers should be cut off as soon as flowers fade; lilacs are very resilient plants.

Planting is best done in early spring or fall.

Sources include Fox Hill Lilac Nursery, Syringa Plus, RareFind Nursery, and Select Plus International Lilac Nursery (Canada).

Also, see our guide to Trouble-Free Lilacs.

- Courtesy of NYBG Plant Information Service