My best friends are 2 girls I've known since first school, even though we don't all get together much anymore (one is at uni and the other works a lot of awkward hours) when we do it's like nothings changed between us, we can still talk about everything and anything and just be proper high schooly again! They're the sort of friends who (hopefully) I'll never lose, even to uni, marriage, moving away etc!
My worst friend was this girl who at one time was my best friend and who, I think, had quite a few self esteem issues and was constantly putting me down, and not behind my back either, in front of other people! One time I just snapped and had a massive go at her and we've never been the same, I'll talk to her if I see her and she does seem a lot nicer now but not so much that I'd try and regain the best friends status we had!