As a general rule, I never eat mammals that are omnivores or carnivores, so pigs and anything higher in the food chain are out. Veganism is a little unrealistic IMHO, but you can certainly eat these in the following order from most to least and try to stay vegetarian as much as possible:
* diary products - eggs, milk, cheese
* shellfish - clams, shrimp
* fish - tuna, salmon
* foul - chicken, turkey
* beef
If you look at beef as sacred (as many cultures do), then the death of one cow can feed countless people. In terms of deaths vs nutrition, it's probably the best meat source. Unfortunately it has environmental consequences, for example the desertification in the west is due to cow and sheep ranching. Usually my only serving of meat each day is a turkey sandwich. Then a cheeseburger once a week for recovery. And I weigh 260 pounds, coming up on a 300/400/500 bench/squat/deadlift so if I can eat meat sparingly, anyone can.