In terms of food you can buy, salads are the best health recovery per money spent.
Otherwise, I like to get Foraging 12 and collect tons of iridium quality salmonberries and blackberries. Also, if you've gotten to Ginger Island and gotten three hearts with Linus, you can make quite a lot of sashimi with the mussels you find there.
This may be an unpopular opinion, but if you have the Artisan profession, I'd advise against using gold quality cheese. You'd get more health recovery from selling the cheese and using the proceeds to buy salads.
Gold quality cheese gives 101 health. With Artisan, it sells for 483g. Salads cost 220g from Gus, so with one gold quality cheese you can buy 483/220 = 2.2 salads. The salads give 50 health each, for a total of 109.8 health. Of course this is only a small difference, so feel free to use cheese if you'd prefer (it's probably less hassle), but mathematically salads are better.
Actually, looking at the numbers again I've realized that iridium-quality blackberries are also slightly better sold to buy salads, rather than eaten, if you have Tiller and Bear's Knowledge. Good to know.
Salmonberries and sashimi made from mussels are both better eaten rather than sold - in fact salmonberries are by far better than any other food anyone has mentioned, in terms of health returned per g.
Personally, I like to have a large stack of food that heals a moderate amount of health (like salmonberries or salads) and ~5 things that heal most or all of my health (life elixirs, pink cake, iridium quality purple mushrooms, gold cheese). The big healing items, even if they're mathematically worse than salads, are nice to have when I'm super low on health and enemies are about to hit me and I want a bunch of health all at once. This is especially true in multiplayer when enemies don't pause when you eat. But I don't want to only have large healing items, because personally I find that I start worrying about "wasting" the health, and then I don't eat until my health gets really low, and then I die by accident. If I have smaller healing items, I can just eat whenever my health gets below half.
Foods that only heal a small amount of health (e.g. regular quality salmonberries, cave carrots) are technically workable in singleplayer if you have enough of them, since you can keep eating the food to pause time until you've healed up enough. But if you're willing to sit through the eating animation as your character slooooowly eats 15 cave carrots in a row, well, you've got more patience than I do.