mockorange/philadelphus

Physocarpus blooms are creamy white and the foliage of some of the new varieties, 'Diablo', 'Summer Wine', is a stunning purple.

Of viburnums, probably the most beautiful are the doublefile viburnums,'Shasta' or 'Mariesii'. They get about 6-7 feet tall but twice as wide. If you want berries as well as flowers, try V. dilatatums. Get two of one cultivar, say 'Cardinal Candy' and one of a pollinator, say 'Michael Dodge (yellow berries) or 'Erie'. Another good combo might be two V. 'Allegheny' (semi evergreen) and one 'Mohican'.

Does it matter to you if the plants lose their leaves in the winter (that is, if they are deciduous)? Many hollies are evergreen but some are deciduous, so if you "like their look", just wondering if you had something evergreen in mind.

Right now my rose of sharon Diana is in full bloom. Each spring I cut back to about 5' and each year it grows to over 10' just covered with blooms. For spring bloom I recommend viburnum...can't beat the hardiness or the aroma.

Evergreen would be nice (it is at the side, but front of house - nice to have some winter interest) but not a necessity. I was thinking of evergreen hollies. Thanks for the suggestions - I'm off to check them out online :)

How about Rhododendrons?

not evergreen but bottlebrush buckeye is a native shrub which can get that tall and has showy white flowers in early summer. also consider oakleaf hydrangea for attractive deciduous foliage and white flowers at a smaller mature height but may not be hardy to zone 5, although 'snow queen' is reported to be.
is your site sun or shade? the two above appreciate some shade in summer; at least they do in my hot zone.

I'd 2d Hydrangea quercifolia 'Snow Queen.' If you are truly borderline Zone 6, I wouldn't worry about hardiness. I never winter protect mine, but it is next to an East-facing wall. But then I'm Zone 5a. The only thing that might not work for you is that the flowers turn pink as they age. Have you considered a shrubby Magnolia?

Bridalwreath Spirea ...

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Very densely branched ... and so provides privacy in the winter too.