Never re-heat pizza in the microwave. 400* oven for 20 minutes.
Perfection.
Screw that. Take a skillet, put the stove on low heat, let the skillet heat up, and then put the pizza in there. Cover it. The skillet acts as a stone, evenly re-heating the pizza, and the cover keeps all the moisture. Let it go for 12 minutes (or longer, depending on the thickness of the crust), and enjoy. Delicious.