What would be the healthiest breakfast?

One that is well-balanced. You need something like that to sustain you. Some fruit as your sugar. Protein and calories. And some carbs and fats.

I think oats with all kinds of fruits,honey and proteic natural pumkin powder.
Omlette with yogurt and avocado plus olivs and chiken or turkey.

In my opinion healthiest breakfast contains some amount of your daily nutritional requirements.

But sometimes/most of the times cooking breakfast on a daily basis is not an option so, you can choose a healthy base food like oats and give it different varieties of toppings for different days like plain oats with milk/curd as base and then dry fruits as toppings or fruits as toppings or veggies as toppings.

Or you can have dry fruits/trail mix as a light breakfast. When you need breakfast on the go.

Having anything in breakfast is much better than having nothing for breakfast.