WHAT SHOULD WE EAT ? may be biaised).
But my conclusion is that fruits are the only ideal food for humans.
Fruits ? Vegetables ? Animal products?
This post is mostly about wether or not we should eat vegetables. I'll let you judge by the links provided (but search for other sources too, for those here

http://www.13.waisays.com/plants.htm
http://www.fibermenace.com/
http://raypeat.com/articles/articles/vegetables.shtml

So these are 3 links provided by someone who eats only raw meat and raw animal products (raw eggs, milk, butter...) telling me that plants are toxic because 1) fibers are not edible 2) most of vegetables contain built-in pesticides/insecticides to naturally fight against predators/to prevent from being eaten.

From the 1st link :

We are not pigeons.

We are not cows.

Therefore we don't have a crop to digest cereals, nor 4 stomachs to gradually digest grasses or leaves.

Right... and are we tigers ? Are we lions or hyennas? No, therefore we don't have the special enzymes and short digestive tract to degrade animal proteins.
We are primates, and like the anthropoid apes we are designed to eat fruits. Most apes eat a diet composed of 99% plant-based food, (most calories come from fruits, the rest from greens) and 1% animal food (the insects on the greens they eat).

My opinion about fibers: I don't think they are harmful to health in adequate amounts. Just because they are not edible doesn't mean they are harmful. They are not useful to digestion itself (unlike many ignorants and dieteticians tend to say or think) - in fact, they slow down digestion and make it harder - but they are good for afterwards : to help get rid of the other wastes (cuz fibers are wastes).

My opinion about the self-defense elements found in plants to protect themselves against species who wish to eat them... Ok, so cows and other herbivores have the digestive system to deal easily with those subtances but we don't ? Why not? 4 stomachs ain't necessary. We have 2 stomachs (1 stomach in two parts) unlike carnivores, who have only one simple stomach. I think we can digest grains and vegetables quite efficiently, but not necessarily optimally (worse with meat and animal products. But we digest fruits optimally).

We have the pro-meat/anti-vegans people who say those molecules are harmful to humans. On the other hand, I've heard from vegetarian dieteticians that those defenses of the plants against insects, microbes and viruses and beneficial to us when we eat them; when we eat those plants those same molecules are then protecting us from viruses, etc, rather than harming us.

Some people say also that plants contain too much insecticides/pesticides sprayed by humans. Not if you buy organic products. If you eat animal products, what you eat is even more toxic because it stored much more insecticides in the animal flesh, after years of intensive feeding.

Those phytochemicals are used both as ingredients for insecticides as well as making supplements supposedly scientifically proven to be effective for their potential health-promoting properties on humans.

Whatever if vegetables are good or bad for us, I don't think some greens everyday can harm us, especially if you choose the ones with no 'harmful' molecules, but I think the optimal diet for humans is fruitarianism.
For those wondering if only fruits can provide enough proteins, you can visit this website for infos. http://www.thefruitpages.com/sugar.shtml We can also add a few nuts and seeds to add some proteins.

I think that the ideal, and natural diet for humans is a low-fat raw vegan diet, has presented in the book 'The 80/10/10 Diet' by Dr.Douglas Graham. It consists of mostly fruits, and raw vegetables, with small amounts of seeds, nuts and avocados.

What's funny is that the diet suggested on the 1st link provided by the anti-vegan guy, is a mostly fruit diet. All meals are fruits, and some raw nuts. The only thing which is not fruits is 1 or 2 raw eggs per day.
http://www.waisays.com/suggestions.htm