Woe is you! you’ve been cursed with severe allergies to all foods but one.
Which one food would you suffer least on? Which food would you survive longest on?
I vote salmon. It is rich in many essential vitamins, fatty acids and minerals. Protein can be converted to glucose and fat can be converted to ketones for energy, Though the total lack of carbs would slow the brain, I think I’ll eventually die of scurvy due to total vitamin-C deficiency.
There is no “perfect” food, so no matter what you choose sooner or later you’ll succumb to some manner of nutritional deficiency, in which case I’ll just go with something I could never tire of, which would be cheese. Has the benefit of existing in myriad varieties so it would never get boring.
I guess you’d have to define “food”. Because if I could have nothing but pizza - I think there’s enough potential varieties to sustain me.
Pizza - is pizza a food group?
I will ride out the time to my demise with fresh sourdough bread.
Peanut butter
Potatoes. My Irish ancestors are grinning.
Vitamins aren’t food, technically, so supplements will keep me going.
Can you choose a dish that has a lot of different ingredients? If so, I’d probably choose bibimbap.
Peanut butter.
If we can’t take one that’s already been mentioned, then Fried Chicken.
I’m not worrying about nutritional issues as I’m not worried about becoming allergic to all other foods except one. I mean, if I choose fried chicken, that implies that I’m also not allergic to flour or seasonings or milk or whatever else I decide to use in its preparation. 
Sweet potatoes. They can be sweet or savory and are quite nutritious.
That’s easy: Pez. Seriously.*
*10 points if you get the reference.
Yeah, even the carb-hating paleo folk love eating sweet potatoes
Chili.
No doubt about it.
(Can I drink beer with that?)
Cherry pez?
Can “cheese” be considered one food? If not, I’m going with eggs.
I think “food” indicates a single ingredient or category, vs. dish.
Cheese is a food. Bread is a food. Tomatoes are a food.
Pizza, however, is a dish.
Venison.
I could eat deer steak every day. I already do when I have it. Woe is me, those long months of summer and spring after it’s all gone from my freezer.
now that i’m avoiding red meat, i’ll go with chicken.
Peanut butter and strawberry preserves sandwiches. On wheat toast.
I was going to go for lentils. They’re pretty much a superfood: high in protein while low in fat and calories, and chock full of fiber, vitamins and minerals, including calsium and iron. They’re also extremely cheap and super easy to cook, for what that’s worth. If I could have some rice as well, I could probably survive pretty much indefinitely on not much else (large parts of humanity has).
However, I had lentils this whole week, and now I’m sick to death of them. So that’s right out. Ask me again next week, I guess.