I get a lot of questions about lilacs as soon as they start to bloom in the spring. There must be something about the flowers and the scent of the flowers that people really like. Around here, I generally only see three types of lilacs; common lilac, Miss Kim lilac and Meyer lilac.
The common lilac (Syringa vulgaris) is an old-fashioned shrub, and the first of the lilacs to bloom in the spring. It gets rather large and gangling, and isn't a shrub I would put as a focal point in the landscape. I would mix it into a shrub border so that after it blooms, it will blend in and hopefully you won't notice that it will always get powdery mildew on the leaves. I have a white-flowering common lilac, and it is part of shrub border.
The Miss Kim lilac (Syringa patula 'Miss Kim') is a much better species to plant . It blooms a few weeks later than the common lilac. I think these work well as a foundation planting, as they also stay more compact than the common lilac, and don't get powerdy mildew on the leaves. I have 3 on the west side of my house that are about 5 feet high and very fragrant when blooming (See picture of American Standard Hoe, An Old Standby in My Garden Pictures to see what mine looked like in bloom last year).
The Meyer lilac (Syringa meyeri) is also a good lilac for a foundation planting, and grows to only about 3 feet tall. I also have some of these. I think the flowers have as much scent as the common lilac, and like the Miss Kim lilac, it blooms more in early to mid May, after the common lilac.
I also have a Japanese Tree Lilac (Syringa reticulata 'Ivory Silk') in the front yard. It is a small tree, and blooms with white flowers in early June.
One trick I use on all my lilacs (even the tree) to ensure good bloom the next year is to cut off all the old blooms. This forces the plant to put its energy into becoming a good, strong healthy plant, producing buds for next year, and not into producing seed for the next generation. It can be a bit tedious to do, but I think it is well worth the time.
I hadn't thought about all the lilacs I have, but I guess I do have quite a few!