for me Worship of God (Alah).
I am also trying to find an answer to this question. Let me know if you can find it first.
You can choose one for your life, or just go with the flow!
Or you can try to think very deeply about why any particular purpose would be objectively better than any other. Then you'll probably realize that there's no objective reason, it's all your choices and assumptions if you dig deep enough. It's fairly depressing, best not to think of it if you ask me!