Red tip Photinia is what I suggested to our kids, who live across the circle from us...you can see 1/2 of their entire back yard from 30 ft away....really drops off badly.The photinia has a really fast growth rate...easy to keep shaped...grows to about 30 ft, I think. Really pretty shrub.

I pondered this question for a long time, in 2007, I ended up investing in about 60 Emerald Thujas that were supposed to grow very quickly to 20 feet. They have been very slow growing and are still at only 3.5' tall :-(.

If I had it to do over, I would plant small trees every 15' with shrubs beneath them. If I had done that in 2007, I would have complete privacy by now.

It is not for nothing that red tip is the hedge shrub of choice where it does well. I read, though, that it is hardy only to zone 8. Waxleaf has fallen from favor, and is considered unimaginative, but it does grow like the dickens, and will withstand neglect well. Much more hardy than hollies.

Red Tip photinia would certainly be the fastest growing, but they are susceptible to fungus, and just when they get to the right size they may get it and it will kill them - there is no cure and then you'd have to start over. Chinese photinia might be a better choice, but they don't grow as fast. Yaupon hollies grow pretty fast, and although crape myrtles are deciduous, they grow fast too. You'd have the winter with no leaves, but the bare branches would offer some privacy. Maybe look at wax myrtle too. Whatever you decide on try to get the largest specimens you can find and afford. They will get to the height you need faster, and you won't be as prone to planting them too close together.

I've lost one red tip, out of 20, to fungus in the last 19 years.