A fresh croissant is a divine gift, but a stale/soggy croissant is absolutely pointless.
Most baked bread-type goods are like that.
Also, most seafood. Shrimp fresh off the barbie are wonderful. 20 minutes later, not so much.
French fries.
Thread relocated from IMHO to Cafe Society.
Pizza. I don’t understand why people thinK cold or microwaved leftover pizza is acceptable. Order the amount you want. If there’s extra eat it while it’s still warm, it won’t do you any harm. If you absolutely cannot take another bite whilst it’s still warm then throw it away and remember NOT TO ORDER SO MUCH PIZZA NEXT TIME!!
No, no. I enjoy cold pizza (refrigerated overnight, not left on the counter like a frat boy would) as much as I enjoy fresh pizza…maybe more. Reheated - never. Cold - hell yes.
Fat in baked goods helps retard the staling process, so any baked good made without fat will start going stale as soon as they cool down. A genuine Parisian style baguette is practically inedible within a matter of hours. However, you can salvage them by making them into tasty croutons, French toast, bread pudding, etc.
Macaroni and Cheese. Yes, the stuff out of the box is crap, and it doesn’t reheat worth a shit either.
But even the delicious, fancy, gourmet cheese, baked stuff is difficult to reheat. It’s hard to get it anywhere near a point resembling its fresh, on-the-table goodness.
Nothing worse than day-old Guacamole.
Spam Musubi is awesome when fresh - the fried spam hot, the nori toasted crisp. Once it cools off it’s still good but not transcendant.
Red beans & rice from Popeye’s. Good when fresh, totally inedible after being in a fridge overnight.
Salad.
Most cold sandwiches.
Onion Rings
Spring rolls
Pizza reheated in a pan on top of the stove is almost better than fresh. Just heat up a nonstick pan, throw in a couple slices of pizza. Flip it over so the cheese side is down for part of the cooking process (really, it works) but also make sure to leave the crust side down long enough to re-crisp the crust.
Reheated (smoked) brisket is just awful.
Microwave - no. Cold or reheated - sure! And some pizza only comes in one size, so ordering less is not an option. Besides, cold pizza is its own joy.
Not reheating brisket? 
I’m sorry, but I can’t eat 13 lbs. of beef in one sitting.
It’s the avocados. They just don’t keep.
Wow. On Mondays a bar I visit has half price pizzas to pull in business. I always buy two or three. Pizza for dinner Monday, then breakfast at home and lunch at work on Tuesday (I share) and sometimes on Wednesday as well.
I heard someone say that if they come out with a none browning avocado, GMO food would become acceptable overnight. 
Depends how you do it. If you microwave it, agreed, it is bad. If you heat it gently in the often, it is a little better, but generally dry and not great. However, if you vacuum seal it and heat it gently in a hot water bath, it is pretty good. Not as good as fresh, but I will happily eat it.