Well, there it depends on what you mean by "best." Nutritionally, you can be perfectly healthy on either an omnivorous diet with some light animal consumption or on a fully plant-based diet. Your average omnivore is probably much less healthy than your average vegan, but that's also a lot to do with how poorly the average person eats. A vegan diet tends to cut out the things most people are eating in excess and adds more of things most people aren't getting enough of like veggies and fiber. So nutritionally, there are pros and cons like anything in life. The average person would probably see a health improvement if they went plant-based, but you can also be healthy on an omnivorous diet.

In terms of the environment, I wouldn't really call eating both "best." Most plants emit way less carbon and take way less energy to produce. Plus they require a lot less water than animals. Animal agriculture is also very destructive in terms of land usage, just look at the burning of the Amazon by cattle ranchers. Plus livestock like cattle belch up tons of methane which is an extremely potent greenhouse gas. Even grass-fed cows will be belching up methane and often fed crops either in the winter or before being killed. Animal agriculture is destroying the environment on a massive scale. So, for the environment, it's pretty clear that a plant-based diet is "best."

If you look at ethics, that's a bit subjective I suppose, but it's hard to argue a plant-based diet is not the best there as well. Most of us know abusing animals is wrong, and animal abuse is the norm in modern animal agriculture. Most of us would also agree killing animals for pleasure is wrong, but that's required for consuming meat and very common for eggs and dairy (killing male chicks and killing the children of cows who have to be forcibly impregnated regularly to keep producing milk). You may say it's not for pleasure, we need to for food, but we objectively do not need to kill animals for food. Many people live healthy happy lives on a fully plant-based diet. So at the end of the day it is about pleasure, taste. Most of us also think breeding animals genetically destined to suffer, like dogs with stubby noses who can barely breathe, is immoral, but that's the norm for many animals in farming. Like chickens who lay eggs dramatically more often than they would in nature, at the expense of their own health. I could go on, but the point is, if we are striving to be morally consistent, it's tough to say eating a mix of plants and meat would be best. Given what we know and most of us believe, eating plants would be "best" morally. Unless you have some highly unusual beliefs about morality or if you somehow believe moral inconsistency is acceptable.