http://en.wikipedia.org/wiki/Thylacoleo
http://en.wikipedia.org/wiki/Diprotodon
As well as Red Gazelles and Aurochs as I feel we as a human species are responsible for their extinction, and they played such an important role in early human populations.
Polish (as well as some international) breeders and geneticists are attempting to breed back (reverse evolution) Aurochs from modern cows, which I think is a super cool project.
I'd also love to see a Dodo brought back from extinction.