Hi Rebecca,

The only plants I know that deer don't eat are invasive species and I can't in good conscience recommend those! 

I have seen 'Green Giant' arborvitae be successful in heavily deer infested areas of Oakland County. For some reason, they are not as palatable as other selections. They are not cheap, though, and I have heard that they are wider growing than other varieties. When they are happy, they grow quickly. I read one account of a 'Green Giant' hitting 40ft after 10 years. Generally speaking, the native species arborvitae (Thuja occidentalis) will ultimately grow to 50-60ft tall. 'Green Giant' is a hybrid of two species and may get larger. Here is an article from Penn State on different types of arborvitae. 

If you must use a single plant--not a privacy fence--I would probably go with 'Green Giant', but I would plan for them to grow large in 10-20 years. I might plant them closer together, for instant gratification, but know that in 5-10 years I will need to cut down every-other-one. 

If uniformity is not an issue, I would think about a mixed hedge. Maybe 'Green Giant' placed 15ft apart with the gaps filled in with shrubs like smoke bush, lilac, and spirea. Once the arborvitae fill in, the shrubs would be removed. 

Please note that small trees and shrubs can be damaged by deer rubbing on them in the fall. You should consider caging new plants in late summer to prevent them deer from rubbing and breaking branches.

I would recommend planting in early fall. Generally speaking, trees and shrubs do better in the fall because they are less stressed by the summer heat. If you plant now, you will have to water them at least weekly throughout the growing season.