The Love of Your Life is the one who will always own a piece of your heart no matter what.
So truthfully, without any bias, I will have to say Angel.
True he was her first love and she was naive, but Buffy was never a child in the true sense of the word. Buffy has two true loves, Angel and Dawn (in a sisterly way of course) And I can't fault her for that b/c I know how she feels.
I broke up with my ex-boyfriend and first love six years ago. We went through a lot together and due to circumstances beyong our control we can not be together. I have moved on and had several relationships, and I am now engaged to a man I love dearly, the man I plan to spend my life my life with. But I know deep in my heart that I could never love anybody the way I loved my ex, I know I will never feel anything so pure and strong again. Does that mean I don't love my fiancee? Of course not. But there is a part of me that will always belong to my past lover, and that no one will be able to touch.
And for Buffy that's Angel.