A tall shrub or small semi-evergreen tree with silky-silvery foliage and bright yellow cone shaped flowers in July that are pineapple scented.

Morrocan Pineapple Broom
other than that NO idea -- Ken

How about a beech hedge, there are some lovely vareties, some variegated

At the side of our property is a shrub hedge...... mahonia, kolkwitzia, ribes, hardy fuschia,hawthorn, dog rose, rosa canary bird ,forsythia, cotoneaster, to name but a few, always interesting, gets cut back once a year.....the sparrows especially love it, oh theres a choisya ternata in there somewhere as well.......

For an evergreen hedge yew would be my first choice although it is a bit dull. Hollies can make lovely hedges and there are some lovely variegated types.

Viburnun tinus is evergreen and makes an 8 foot hedge and has flowers from late autumn for 3 months or so .

Choisya ternata is also evergreen and has lovely white flowers and scented foliage

I would go for a beech hedge, personally my favourite. A bit slow to get going but looks great all year round. Easy to care for, no thorns and good for the wildlife and native too. We set ours about 3 years ago and its a proper hedge. :-) A good time to set a bare rooted hedge now too. We also have mixed hedges of hawthorn, maple, viburnum etc. which are great but the beech looks neat.

I'd go for native mixed hedgerow. hawthorn, blackthorn, viburnum, hazel, alder, elder, beech etc. supports lots of different wildlife. this is always better than a single species of hedge plant.

The above doesn't give privacy all year around though..if that is what is required

It does depend what effect you want and how much room you have, eventual height, whether you want privacy, security (thorns), wildlife friendly, flowers, evergreen etc. Can you be more specific? What do you mean by arboretum quality, And a caveat - smaller plants establish much more quickly than large ones as well as being cheaper.

You could put Beberis it is spikey and if you have people walking past it stops them from breaking bits off it