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

Ideally, I would have preferred if it was 2-3 years difference so that theu would grow up close and have more topics to talk about. In reality, my kids are 6 years apart. I thought they wouldn't be interested in each other but I was so wrong! It also helps that my elder girl is very eager to help out with her brother so I can get some time away doing chores if I need too!