(John Hartley - a culinary patriot eating & cooking in Northwest England)
Too wide a question, as framed, for me. It’s the “must order”. So, looking at your list by way of example, I love sweetbreads but they are never going to be a “must order” just because they are on the menu. It needs to be a restaurant where I know the chef has the skill to not overcook them - which usually means somewhere upmarket.
Invariably tongue from a Mexican menu. Tongue sandwich at a proper deli if we’re not requesting two slices of rye containing chopped liver or whitefish salad, or a reuben. While hopefully checking the Bistro Jeanty (Yountville, CA) menu, we sadly noticed that a decades-long favorite of lamb tongue and potato salad vanished – what a difference a year makes.
Mostly seafood. Crab, clams, oysters, mussels, abalone, calamari steak, halibut, sole, even fish & chips. Swordfish is another story as I have yet to have it as good as I can grill it at home (provided I can get fresh swordfish).
In Thailand I ordered som tom thai with small salted/fermented crabs every time. Anywhere. Street stalls, restaurants, food courts, from a lady on a long slender boat at the floating market.
Thai restaurant (“Salathai”) in Vancouver, Canada only chicken basil cashew. Every time I tried something else I regretted it so the next time I ordered chicken basil dish again. Been there about several hundred times or more and never ate the rice but they still asked a couple of times if I wanted any. They never had anyone who didn’t eat rice with the dishes.
In Vietnam I asked scooter drivers where to eat Bun Rieu Cua (tomato soup with crab roe), and rice sheets filled with mince. They took me to their favourite spots. Pho is everywhere there. In Vancouver I ate pho in restaurants almost every single time. They were not competent at other soups. Had good rice sheet rolls in Vancouver. The shop manufactured the rice sheets for restaurants and they also served this dish in their shop.
In Sicily I must eat pickled aubergines and grilled aubergines kept submerged in olive oil.
In Austria it has to be Speck anything.
Regional foods when in Germany. In Mittelmosel at the moment it’s “Saumagen”/stuffed pig’s stomach, the “national” dish of Rheinland-Palatinate.