Maybe in between 1 and 2... on the biggest natural crack as stated.
If it's frozen, go for 1, but if not, sap wood, in my experience, can be too soft to transmit efficiently the energy of the strike. It compresses and absorbs.
Otherwise, by hitting towards the edge, the least amount of wood needs to separate to get a split started. In otherwords, if the edge separates on one side, the other side can remain attached, therefore making it easier to get the split started. If you strike in the middle to split, the whole round must separate for the middle to separate. Too much work to do all at once, IMHO.