Hi Aurora!To me there is not one definite answer to that question. And which soulmates do you mean? Is one more important than the other? And does that matter? All relationships are equally important as you have attracted them into your life and are necessary for growth or so you have chosen it to be. Karmic soulmates, companion soulmates? Soulmates do not walk the linear rules and it all depends what contract they have between each other. Sometimes a companion soulmate or someone from your soul family (we actually all are family) comes into your life for a really short time to transform something in you. Frequently it is about self love. When the person finds your happiness important, he or she only wants the best for you, he feels happy because you feel happy, no needs, no strings attached. When he loves you without expectations, just for who you are NOW. That's a big chance he or she is your soulmate.
In my experience in synastry a lot of outer planets to personal planets, nodal contacts (conjunctions to angles, personal planets or squares (action) to personal planets), Ascendant/IC contacts.
Your feeling defines it all. How do you feel when you meet this person? You just know, you don't wonder.
Your question is almost impossible to answer. There are no rules.
Vertex contacts to points or personal planets.But that's just the meant to happen aspect, doesn't define anything and could be any relationship.
The best soulmate you can meet is YOU! Hahaha.. The relationship with YOU is to me the purpose of life. Hope this helps.
Why do you ask?