Scalloped potatoes are supposed to be fried in butter the next day. It is their telos, their purpose, their end.
Fried rice is better the next day. I think sesame noodles are, too. A lot of stews are better reheated, it goes without saying, really, that soup does too.
I make a stew with a lot of spices, especially Indian spices, added. It is so much better the second day! After the tenth day, it gets a little spooky though.
lol this will come-off as weird: (and not exactly a leftover)
Tomato soup. The more times you reheat it, the more concentrated it becomes. shiver of delight
Beef bourignonne, brisket, my grandmother’s sweet-and-sour meatballs…pretty much any Northern or Eastern European peasant cuisine main dish.
Pie for breakfast the next morning. It may not actually taste better than it did fresh out of the ove, but the guilty pleasure of eating pie for breakfast more than compensates.
Chili, any sort of soups and stews, spaghetti sauce, meatloaf, a lot of rice dishes, especially when there’s something creative done with the reheating (fried rice makes the best rice burgers I have ever had), potatoes and gravy (the texture is just so much better the second time).
Not exactly what you mean but whenever you cook duck you should retain all the rendered fat so that next day you can have a massive breakfast fry up of eggs, bacon, hash browns, sausages and french toast. Unbelievable how much better it all is cooked in duck fat.
Spag Bol if you have mixed it all up the first time around (as you should) with a tablespoon of water to steam it in the microwave.
Fried spaghetti! Cold cooked spaghetti, fried up with butter and Parmesan… mmm, comfort food.
Rice pudding. Last time I made it, it was fantastic warm out of the oven and lovely straight out of the fridge the next day. That pudding didn’t last long.
Anything involving lots of spices and herbs that need time to blend.