Only children of course. Here's why:
(1) Only children never have to fear being ever dethroned by a succedent sibling or siblings. They will at the center of their parents' universe forever or at least until there are grandchildren. Only children will never be discarded & cast aside like oldest children are. Also they will never be overlooked or cast into shadows like middle children routinely are.
(2) Only children never have to vie for their parents' attention & love. They have their parents undivided attention & have free access to their parents. They know that their parents are there for them. To only children, their parents are their support system. They also relate to their parents in ways that children in multichild families don't have or will never have the opportunity to do so.
(3) Only children are free to be individuals. They are inculcated to have a very high sense of individuality by their parents. They are valued for themselves. Only children don't have to be relegated to rigid sibling roles as children in multichild families usually are. They also don't experience parental favoritism which is also rife in multichild families. As a result of being valued as individuals, only children are highly confident & are quite sure of themselves.
(4) Only children are quite comfortable in their own skin. Because they have no siblings, they are quite comfortable being alone. This aloneness aids in their imaginativeness & creativity. Only children are the most imaginative & creative people around. They are quite adept at entertaining themselves. This also makes them to be quite independent & not as needy as those from multichild families who are uncomfortable being alone & must always be surrounded by others. Only children AREN'T afraid to be alone. In fact, being alone allows them opportunities to explore themselves & their environment.