What is the best age gap between first child and second child?

Two, three years of gap is good as as they grow they can have company in each other. The elder one is not too big not to understand and relate to the little one and can help the little one in many ways, at home or school. But, I have seen my sister-in-law has two kids , a girl and a boy and the age difference is of 6 years. And I have seen how nicely the sister takes care of the brother. My sister-in-law is working, so the sister takes care of the brother when mum is not around, helps him in his studies, even feeds him if need be. The younger brother has picked up reading habit from the sister and at the age of 6, ( I feel amazed) he is a voracious reader. Though, 2, 3 years of difference seems ideal, but the pros of bigger age gap too cannot be denied.