Taronga would be mine pick. Closer than Featherdale and arguabley the nicest setting for a zoo anywhere.

If you really want to see Kangaroos in the wild you don't really have to go too far out of Sydney. I remember when I was kid going camping at the Basin, just across Pittwater from Palm Beach (northern suburb of Sydney), and the campsite would have lots of Kangaroos and Wallabies in the mornings and late afternoon. But I'm sure there are plenty of other places to see them too.