I've been with my current best friend many times in the past. In this life, we met our freshman year in high school. I consider him like a brother to me. He was my only attendant at my wedding. We've lived on opposite coasts for several years, but have a bit of a "psychic" connection--I usually know when's he's about to call me, and vice versa.
I believe we were together most recently three (for me at least) lifetimes ago. We were a married couple during the period of the American Revolution. I was able to confirm the existence of this particular past life of mine, because he had a military record. Interestingly, in that lifetime we lived in my (and my friend's) home state in the northeast, which may explain our rather irrational attachment to the place. Although my friend moved away over 10 years ago, he often mentions his desire to move back. He's agnostic on the topic of past lives. I've told him about this particular life and the confirmation I've uncovered, but not about our relationship with each other. He was interested, but didn't express any memories of his own at the time.
According to information I've received through regression/meditation, I suspect he may be the person in my life currently with whom I've shared the most past lives (or at least, lives which are of current significance). I do not consider him my "soul mate," rather a beloved long-term collaborator.