Microwave - NEVER
Cold - on occasion, with discretion
Toaster Oven - 400 degrees F middle rack until cheese is just about to begin to bubble, then immediately take it out. It's restored. May take a minute or two longer for slices with toppings or non-tomato sauces (marsala, buffalo, etc.)
As an ingredient in other dishes - omelet, hors d'oeuvre, etc. - acceptable, if cut into strips on a bias first.
Slices over three days old - throw away, the magic is gone. Also the only acceptable time to throw pizza (Breaking Bad notwithstanding), otherwise always lay slices flat gently with the crust away from you.