Mace is the outer membrane of the nutmeg plant, so ground nutmeg would make for the most similar tasting substitute. Other common substitutes include ground allspice, pumpkin pie spice and ground ginger.
Mace is supposed to have a stronger flavor than nutmeg, but since they come from the same plant, their flavor is similar. Allspice is another recommended substitution. These are equal substitutes, which just means that the amount of mace that a recipe requires can be replaced with an equal amount of the substitute spice. The other replacements - pumpkin pie spice, ground ginger as well as ground cinnamon - are going to produce a less-similar flavor than the nutmeg or allspice would produce.