For me it is Carla- best friends since the day I moved to Chicago in 1974- so this coming summer it will be 32 years! She is still there, but we are still close and when we talk- it is like nothing has changed.
If a friend can be someone you were close with once, and now talk to only once every couple of months, or even once a year, then my longest friend would be Joe. I worked with him at my first job in 1987, so that would be almost 19 years.
If by friend you mean someone I talk to on a regular basis it would be Dylan, whom I've known since maybe 90. So only 16 years.
I don't know ANY of my childhood or high school friends anymore. In fact, I pretty much wrote them all off the day I graduated.
My longest friendship (not counting Mz, we've been friends since day 1!) would have to be with my friend Jessica, 15 years and counting.
Before that was my friend Lisa, we had been friends since I was 3, but then my senior year of high school, she decided to steal my identity, and that definately ended our friendship. I haven't seen her since I found out, and trust me, she doesn't want to see me...