I would also have to say coco coir.
Jiffy/peat pucks also typically aren't pH stable. Sphagnum peat is quite acidic (even if it was sustainably harvested), it can be used but I would drench them in water that has dolomite lime in it first to bring the pH up and stabilize it. Rapid Rooters are preferable over both coir or sphagnum peat for cuttings.