The best stage in life for me is young adulthood because you have the legal age and you are able to do things you couldn't when you were under eighteen.
At this stage of your life you can have your driving license and you can have your first car and go wherever you want to go whenever you want without having to ask someone to drive you. Another reason why I think this is the best stage of your life is because you finished the hard last years of high school and you are able to go to university and focus on studying something you really like and something you really enjoy doing and after this, if you are lucky, you can work on something you have studied for and something you enjoy like doing. Also at this stage of your life you can leave your parent's home and go to live near the university with your friends or with new people you meet there, because you have many new friends there, and you start living your life by yourself being independent and doing whatever you want and being responsible.
By saying this, I don't mean that other stages of your life are less important. For example, infancy and childhood are very beautiful periods of your life, but from infancy, we don't remember anything and from childhood the same, we just remember some things we have seen in photos. But anyway, those were times of your life where you just focused on living life happily and not worrying about anything.
From adolescense, I think it's also a beautiful period of your life, but it's a bit hard because you have to focus on studying, but you also want to hang out with your friends, and your parents don't trust you because you are young yet, but it's beautiful because you don't have to worry too much about problems in life, you see your friends in high school every day, you probably will have your first love, and you will experience many new beautiful feelings.