And if you min/max in that way, you're likely going to be introducing some other problem (perhaps one that takes a long time to recognize) into your complex biological system which is accustomed to having a variety of inputs.

Granted, for non-plant protein, small/young sardines are probably the best. Not only do you get fish oil, but you get load of calcium which is difficult to get elsewhere unless you consume a lot of dairy.

Beef and eggs are trickier, because their downsides tend to be related to how they are produced/raised. Also there's the animals in captivity factor which does not scale well (or humanely).

Pumpkin seeds are an amazing source of protein (and some other minerals which we don't normally get much of). Dried, I think they taste horrible. But roasted with a little seasoning, they are quite nice. And they can be added to other dishes to give extra texture.

Chickpeas are good. Peas are good. Cashews and peanuts are good.

And if you consume a variety of vegetables, you can get the complete assortment of amino acids (so usable protein).

There is virtually no way someone could get fat from eating the foods I have listed. If they do, then they have some other underlying health issue or a gut biome that's way out of whack (which most of ours are pretty far out).