Eat 400 grams of fat per day, in isolation, in a sedentary individual, and you will gain weight. You might not develop any acute health problems necessarily. This would be the answer to "most fattening."
Eat 400 grams a day of only carbohydrate and you might not gain weight (1600 kcal/day won't cause this) but you're going to feel like crap, have no muscle tissue and likely be very messed up metabolically. This would be the answer to "most unhealthy over the long-term."
Eat 400 grams of protein a day without any fat or carbohydrate and you will die far quicker than the other 2. This would be the answer to "most unhealthy overall and lead to the quickest death."
The only one to possibly survive (not necessarily thrive) on would be only fat.
Well, the least would be protein. And since your measurements are in grams, not kcal, then it would be fat, due to it being 9 kcal per gram, then carbs, then protein in order of most to least fattening.
Well, fruitarians end up pretty emaciated. People who eat a lot of lean protein end up starved (rabbit starvation). People who eat a lot of fat stay healthy (I'm thinking about people who eat a lot of whale blubber.) So my vote is for fat since eating the other macros in isolation will basically kill you.
protein is the most satieting. followed by fat. followed by carbs.
source - theres a tiny experiment on youtube involving 3 guys eating a protein rich pasta, fat rich pasta, and carb rich pasta...
Insulin must be present to store and utulize nutrients properly, so if you are not elevating insulin by eating either protein and/or carbs, 3600 calories from fat will not make anyone gain weight (unless insulin is somehow being elevated in a way other than carb or protein consumption). So if you physiologically cannot proudce insulin at all, you will not gain an ounce eating just fat.
That being said, I am not a diabetic and am highly insulin sensitive, so eating either 1600 calories from carbphydrate and I'd lose weight. But then again, are we talking fruit or starch? 1600 calories of protein and no fat? I'd get too sick and die of rabbit starvation or renal failure/