I would personally be more interested in how the food makes you feel nutritionally when you live on it than how many calories it has, but that's me. So, I would recommend a wide array of things that combine together to make a good meal. Zucchini is very nutritious, though (I've eaten raw zucchini for 30 to 55% of my food while it was in season, and it felt pretty good, but it wasn't the tiny zucchini that people usually eat). Each kind of fruit/vegetable gives you unique nutrients. I'll bolden the items that I think would be higher in calories, and italicize items I think have special nutritive and/or culinary properties that might be particularly helpful to sustain health (these are just my opinions based on how I feel when I eat them and what I know about them).
Anyway, you might try sunflowers (ones with large heads and seeds), walnuts, plums, pears, apples, potatoes, early starchy winter squash, cabbage, brussel sprouts, broccoli, garlic, bunching onions, amaranth (the leaves and seeds are high in protein), wheat, oats, corn, peas, cucumbers, early melons/tomatoes/peppers/tomatillos fit for your area, other summer squash, cold-hardy mulberries, soybeans (if you're into fermenting), spinach, sorrel, beans, cranberries (which are excellent), etc.
If your area has reasonably hot summers (however short they may be), I strongly recommend growing Armenian Cucumbers (which are technically melons, and not cucumbers, but are used like giant cucumbers). Note that Metki melons are also Armenian cucumbers. They get huge, they're early (you can harvest them much earlier than regular melons, and they do get huge much earlier than other melons, too—at least in my Idaho garden), and they can handle a lot of conditions that regular cucumbers struggle in. While Armenian Cucumbers might not be very calorically dense (although they and regular cucumbers probably have more sugar than a lot of people suppose), they are very refreshing, and in my experience, they hold me over pretty well if I'm thirsty and don't have any water handy (even better than watermelon, in that regard—at least with the large Armenian Cucumbers; I'm not sure about if you harvest them smaller like a lot of people do, since I think they're less juicy then; I'm not saying they're juicier than watermelon, but I'm talking about the thirst-quenching powers of it that make you feel like you've consumed more fluids).
This is kind of off-topic, but if you're looking for life-sustaining foods, you may be interested in lacto-fermenting them, as I've read that you don't need to eat as much food if you include lacto-fermented foods in your diet. It's one way to make pickles, and preserve food longer (in a raw form; the fermenting increases the nutritional value, and the resulting pickles have beneficial bacteria, too).
Dried plums can be pretty nutritious. Zucchini pickles are supposed to be pretty nice. Green onions feel quite nutritious, and bunching onions grow lots of them (Crimson Forest is my favorite, so far), and they're both cold hardy and easy to grow, even from seed. Mulberries feel nutritious to me (especially compared with other berries), but I hear you shouldn't ferment them, whether or not that's true.
Cabbage is a good vegetable to eat in reasonably high quantities (not so high that you get thyroid issues, though). Potatoes, too, but that's a given. :)
One thing to consider is that although some fruits may be high in sugars, if you eat lots of them, you'll likely get diarrhea, a sore tongue and sensitive teeth (from the acids). So, I would look for foods you can eat in large amounts safely, and eat things like blackberries, strawberries and ground cherries in lesser amounts (and/or in pies/jam).
Another thing to consider is the variety of the plant (not just the species). Some of them have unique qualities and are more productive and tasty than others. How you grow your plants can make a big difference, too.
I would look at the mineral contents, in addition to fats, acids, sugars, starches, etc. Minerals help with lots of things, including enzyme production (especially magnesium and zinc).
Flavor shouldn't be underestimated, either. If you can't choke it down in reasonable quantities on a regular basis, you might be in trouble. Nevertheless, there are a lot of culinary ideas that would help out with most and/or all things. Onions, green onions, garlic, garlic scapes, chile peppers, herbs (e.g. basil, oregano, and tarragon) and such shouldn't be overlooked here.
Elephant Garlic is interesting, as it's much larger than regular garlic (although technically, it's more closely related to leeks than garlic).
I'm not sure if it's a real concept, but in the Good Earth, by Pearl S. Buck, the main character seemed to live on nearly garlic alone when he was poor at the beginning of the book (and he relished in it). Eating garlic seemed to be the norm for more poor people in the book. I don't know if that's really possible, as it sounds iffy, though. Garlic is high in minerals and flavor, but I'm not sure about other nutrients.
Potato onions and walking onions might be good things to consider, since they multiply every year, and you don't have to buy more bulbs or grow from seed.
