ponders

I don't know how one would quantify "heartiest".

I guess if one uses caloric content, it's probably something with a lot of oil, since oil is about as calorie-dense as you can get food.

Plus, you need it to "flow", water doesn't have calories, and so you're going to want a liquid to replace water as much as possible.

Oil itself is drinkable, though I'd guess that you probably wouldn't count it as a soup.

How much oil you can add to a soup -- because various recipes I see online do have recipes suggesting "add to taste" -- before it stops qualifying as a soup is probably not well-defined anywhere.

EDIT: That's maybe more-analytical than what you want, pushing the boundaries of what it means to be a soup, not listing some existing recipe. If you're just looking for some hearty soup suggestions, this might qualify:

https://feelgoodfoodie.net/recipe/peanut-soup/

Peanut soup is an African recipe that’s very popular in West Africa, and especially in Ghana. I grew up in Sierra Leone, so the version we used to eat may slightly vary from other regional versions. But essentially, the recipe is made up of ground peanuts (or peanut butter) with tomato paste and spices.

The authentic name for peanut soup is granat soup. Granat is the krio term for groundnut – aka peanuts. While it’s common practice to grind the nuts for making the recipe, any peanut butter you buy works well here. Just make sure there’s not sugar added.

What does peanut soup taste like?

If the idea of peanut butter and tomato paste turned into soup makes you question this recipe, I strongly urge you to get past that apprehension! There’s a cup of peanut butter in the recipe, so it will taste like spicy watery peanut butter. Served over warm rice, it’s a creamy, spicy and comforting combination. It reminds me of Thai peanut sauce and has all the feels of a curry.

Peanuts have 587 calories in 100 grams, according to this database. So probably as much peanut and as little tomato as you will permit to qualify as "drinkable".

EDIT2: Roux, a thickener, is also in significant part fat, and what isn't fat is flour, which isn't too shabby when it comes to calories either. So maybe a roux-based soup with as much roux as you'll permit in before you'd call it not drinkable.

EDIT3:

italian wedding soup

I honestly wouldn't have considered Italian wedding soup to be drinkable, myself -- I thought that you wouldn't permit large chunks of things -- but if you permit in chunks, and the only constraint is that it needs to be pourable, that might open more options, since it'd let the water content be reduced.