I'd say jack too -- but I think it's a mistake to expect one right answer in this sort of case (as with the "proper" collective nouns for various animals).
As RogerK says, there are bound to be regional differences, and good dictionaries are descriptive, not prescriptive: they give the words real people do use, not what we all should use.
Buck & doe are used for a lot of animals which are smallish and rabbit- or deer-like, so they would work OK for hares in areas where there isn't a special word.
Richard