Scientists have discovered well-preserved frozen woolly mammoth fragments deep in Siberia that may contain living cells
Theres your answer right there. THey found a really good sample that may contain still living cells. I am not claiming to know much about the cloning process, but i think having quality material to work with has a drastic effect on the success of the cloning process.
Mammoths may be able to best aclimatize out of all the possible candidate species maybe?
edit on 12-9-2012 by MDDoxs because: (no reason given)