Advise on the variety of olive to plant tends to lead straight away to the question "What do you want to get out of growing olives and how much work are you prepared to put in?"
You have a few options which come down to:
Where are you located?
Are you planting for oil or table olives?
Are you intending to pickle or make your own oil, or sell it as fruit to somebody else?
There are no hard and fast rules, but:
Manzanillo, Sevillano, UC13A6 (Californian Queen), Azapa, Mission & Verdale all seem to do well in the coastal areas, producing premium quality table olives.
The Frantoio family (Paragon, Correggiola), Nevadillo Blanco, Koroneiki & Leccino seem to well everywhere for oil.
However, Kalamata & Hardy's Mammoth have not been as successful. Kalamata seems to be okay in frost free areas, but is a bit of a worry if the temperature drops below -3C.
Manzanillo produces excellent oil, but is difficult to extract and probably not economically viable as an oil olive.
We are still experimenting with many varieties and until we have another 5 years more experience it is quite impossible to predict which varieties are best suited to the Hunter.
If pushed for an answer, I would suggest that oil varieties are best suited to the inland areas to the west of Singleton, and table varieties on the coastal plain around Cessnock, Maitland and towards the Barrington Tops. However, I expect many growers to be able to prove exceptions to this rule.