So according to my first-ever poll, reheating pizza in the oven wins out by a narrow margin over cold pizza, 22-17. A handful of people nuke their pizza (3) and a couple reheat it in a pan on the stove.
For what it's worth, my thoughts on the subject -
Cold: can't beat it for ease, simplicity and quickness. And generally it is pretty good, but some pizzas are better cold than others. If the pizza has a lot of cheese, sometimes it congeals into a solid, unappetizing mass when it gets cold.
Reheated in oven: not bad, but it takes a while and is not exactly energy efficient. Problem is, the best results would require the greatest energy usage. Ideally, you'd heat a pizza stone in your oven at 550 for at least 45 minutes, then throw the leftover pizza onto the stone for a couple of minutes. That's essentially what slice joints do when they rewarm a slice for you, and the result can actually be better (because it's crisper on the bottom) than when the pizza first came out of the oven.
But that's a long time and a lot of energy for a slice or two of pizza. So most of us just put the pizza in as the oven is preheating to 350, or stick it in a toaster oven. It comes out OK, but never anywhere near as good as when it was freshly baked.
Reheated in microwave: OK, I've done this (mostly when preparing lunch for my 6-year-old), but all in all, I'd prefer mine cold. Obvious advantage here is that it's the fastest way to reheat the pizza. But I'm big on texture, and it tends to make the crust soggy. My daughter has never complained, though.
Reheated on stovetop: I came up with this idea on my own, but I've seen it suggested elsewhere. It gives you a nice crisp underside, and it's fairly quick. (Just make sure you use a nonstick pan.) The one flaw is that it's not too good for rewarming the toppings, though that can be helped by putting a lid on the pan. Or try a combination of microwaving the pizza just long enough to remelt the cheese, with the stovetop method, to crisp up the bottom.