There is no hard and fast rule about the best temperature to reheat. It often depends on a combination of convenience and smart thinking.When re-heating it is best to first bring the dish to room temperature before placing in the oven or on the stove.
By "convenience" I mean it depends what your other needs in the kitchen are. If your oven is being used to, say, cook a pot roast at 350F and you need to reheat your potato casserole, stick it in with the roast at the same temperature.
Here's where the smart thinking part comes in. If you have a large casserole or roast, it will obviously take longer for the heat to travel to the center than if you have a sheet pan of small roast potatoes. Big objects should be re-heated at a relatively low temperature (300-350F) so the outside doesn't get dried out or burned by the time the inside is hot. Small object heat through more quickly, which means a hotter oven–perhaps up to 425F–will work.