I have two.
My first best friend is Lesli. We met in 2nd grade. We were in between assignments in class, so we started talking and playing "house" with our trapper keepers (who remembers those?). We sat our trapper keepers up to look like houses and were pretending to live in them. Come on, give us a break. We were 7 years old! We remained friends all through school except for a period of about a year during our senior year of high school. After we graduated and I was married, we became friends again. We can go days or even weeks without talking to each other (although we rarely do) but still pick up right where we left off. She has been through so much with me and I with her.
My other best friend would be Kent. He knows me inside and out. He knows my deepest and darkest secrets. He has been with me through good times and bad times. He has been my rock, shoulder, everything. Kent has been there at my lowest points and pulled me through. He listens when I need someone to talk things through. He has held me when I've cried uncontrollably. He is just my everything. He is my best friend, but so much more.