A: Growing alongside another person and building up a shared history and celebrating each others' victories and comforting each other when things are hard... Building a life with them over the course of years and looking back with a shared sense of pride and accomplishment at how far you've come. Feeling honored to have been able to bear witness to each other. Cooking and sharing meals together. Sharing physical intimacy with a deep level of trust and cultivated compatibility.
Those are all the best thing. There's a lot more best things too. Assuming the relationship is good.