I've been eating healthy for years and I still don't know what the best diet is.
It's complicated unfortunately, and there's definitely individual differences at play too. For instance many people in Africa and around the Mediterranean are lactose intolerant. That's because milk goes bad quickly in a hot climate. And so they eat cheeses and yogurt. But in colder climes, like Russia, where milk can stay milk for weeks, people are much less lactose intolerant, which not only means they can drink milk, but that they have been drinking milk for ages, otherwise they'd be just as lactose intolerant as their ancestors from Africa. So there goes the vegan argument that we didn't used to drink milk!
I try to follow the official recommendations from the nutrition expert establishment, but nutrition science is still in its nascent years, there's so many things we don't know yet. Michael Pollan is a good go-to option for easy generalizable food rules. His one-sentence mantra just says "eat [real] food, mostly plants, not too much"!