Nothing beats leftover pizza! While eating it cold is certainly an option, reheating it can give you the same delicious, gooey, cheesy taste that you enjoyed the day before. There are a quite a few different methods of reheating pizza. Determining which one is "best" is really a matter of personal taste. Here is a quick breakdown of each method, along with a description of how the pizza comes out in the end. You should be able to use it as a guide to decide which method is right for you:
The vast majority of people turn to the microwave for reheating pizza for obvious reasons...namely that it's fast and easy. Unfortunately, however, it doesn't always give the best results. The crust can become rubbery after it is microwaved, which for some people is a deal breaker. However, if you like your crust rubbery (some people do) then this can be a good choice. To get the best results, turn the power of your microwave down to about 50% and heat it for a minute.  Heating it more slowly will generally give you better results than blasting it on full power.
Heating the pizza up in the oven is another option. Just toss it in for about 5 minutes at 350-400 degrees. This mimics the method in which the pizza was originally cooked, and thus will give you similar results. Your pizza should come out crispy with perfectly melted cheese. This completely eliminates the rubbery crust that you can get in the microwave. Keep in mind, however that baking will remove moisture from the pizza. If it is already a little bit hard or dry from sitting in the box all night, you may want to try a different method.
On a hot summer day there is no better way to reheat cold pizza than by tossing it on the grill for a few minutes. Just put it directly on the grate and allow it to cook until the cheese melts. The crust should be nice and crispy, and the cheese should stay gooey and delicious. Not only that, but you also get the added benefit of beautiful char marks on the bottom of your pizza. The only downside to this method is that it is fairly seasonal. Unless you want to be outside grilling in the middle of winter, you may need to look for a different solution for cold days.
After reviewing countless different websites about the best way to reheat pizza, the hands-down winner is heating it up in a skillet.  Just put the skillet on the stove over medium heat and allow it to warm up. Toss the pizza in the pan and cover it loosely with a piece of aluminum foil. Allow it to cook in the skillet until the cheese melts. When done, the crust is perfectly crispy, the cheese is melted to gooey perfection and it is an absolute joy to eat.
This video does a fantastic job of demonstrating the skillet heating method:
Feel free to experiment with all four methods to see which one you like the best. Happy pizza eating!