Think of a food footprint as the amount of acreage that you need to live on. Plants is the most efficient because, well you eat the plants that grow directly from the farm. Meat is less efficient, Fish being the most efficient, then Poultry, then Pork, then Beef. Fish and poultry are more efficient because it only takes them a little while to reach commercial size and they eat less to get to that size. Beef is the least efficient because they take a long time to reach mature size and require a lot of food to reach that size.
That is a simple model. In reality it mostly works, but it doesn't work perfectly. For example, fish is not the most efficient animal protein, insects are. Also, fish and poultry fed on grain are less efficient than pigs that eat food waste and cattle that eat silage and grass, because humans can't eat those foods so, those animals make plant growth more efficient. But insects can eat some of those same foods so a more advanced model is necessary to figure out the most efficient path way for food.
Grain production produces a lot more grass than it produces grain. The grain is what we eat, but it is not very efficient because we only eat a small bit of it. We could add the grass stock and leaves to cattle food, we already do, so this increases beef's efficiency. But if we feed the silage to locusts and turned the locusts into pellets to feed poultry and fish, we would be reducing the amount of grain that both consume, and let's face it, poultry prefer insects to grain, and most commercially farmed fish prefer a meat diet in nature.
Other vegetable plants have similar waste issues, particularly amounts the grains. Some, like lettuce are extremely efficient, but others like tomatoes and other vegetable fruit crops have a lot of plant matter left after harvest. Some of this can be fed to cattle or insects, but some might not and will remain less efficient. Even after we have made our food production as efficient as we can there are other savings to be had.
Food waste. Humans throw away a lot of food. Some food spoils, some is unappealing, and some parts of the food is considered inedible, while still being nutritious. We tack this waste and throw it out. We could compost some of it but some parts of it are not readily compostable in most circumstances. Also compost takes a length of time to be turned into something that can be used in agriculture again. One solution is garbavores, things that eat garbage. Rats will eat garbage but so will pigs. Pigs will eat spoiled food, fruit, vegetables, dairy, and meat too. Pigs and other animals produce manure and while that has to be composted too, it is easier to do that. Indeed, it is possible to inoculate the raw manure with fungus and produce more food as mushrooms.
So now the question is, what is the best diet for the environment and for the health of the consumer? We come to the words of the best diet for humans summed up in seven words: Eat food, mostly plants; not too much.
Eat plants, waste eaten by insects to feed us, or to feed poultry and fish, to feed us with meat and or eggs. Ruffage used to feed goats, smaller than cattle so faster to grow to produce milk and meat. Sadly I foresee that we will likely want eggs and milk products for the longterm future, because cooking requires it and cheese is very pleasurable for most people.
The answer is not what most people would suspect from a longterm Vegetarian like myself. I recognize that if our food supply is to become maximally efficient, the addition of small quantities of meat would be needed.