Ideally above the mast, as the mast will screen the reflector at least partially. (One day someone will invent, manufacture and market a radar reflector designed to work on the sorts of boats we use. Until then...)
In practice most people seem to mount theirs ABOVE the spreaders - often a quarter, third or half way up from there to the top of the mast. Most mount them to the front of the mast, some to the side.
At the spreaders has disadvantage of yet more holes in mast at a similar level to those existing, and will probably look a bit odd. How high above is a judgement of how best to compromise maximum height, aesthetics, and disruption of the flow of air over your sails.