My oldest friend would be Tanya. She is one day older then I am and so our mothers naturally shared tips on toddlers. On our first photo together we are about one year old (I think, I'm bad at age deduction). We were bestest friends until we were six or so, when we got into a fight over teasing my little sister and since then we pretty much didn't talk to each other which wasn't helped by the fact that I was a "nerd" and she was a "class queen". Now she have moved to Germany and we had a couple of chats online. I'm really sad that I couldn't stay her friend.
My close friends change over time, but that's mainly because I'm bad at keeping contact when we no longer see each other often. But I'd say that now my closest friend is my second oldest friend Veronika, who I see as more of a sister, even if we live across the city from each other and meet only several times a year (mostly in summer when we both go to dacha on weekends and live close to each other).