What's the best way to reheat frozen boiled potatoes?
When I have left-over boiled potatoes, I always freeze them and when I have enough frozen potatoes, I pour them (still frozen) into an oven-proof dish, spray some olive oil over them, throw in a branch or 2 of fresh rosemary and put them into the cold oven and then set it to 150°C and let them heat up that way.  (until the tip of a fork pushed into the middle of the largest potato comes out hot.)
Then you'll have perfect par-boiled baked potatoes!
Boiling or microwaving them makes them too soggy for my taste...