You look forward to seeing them, they make you laugh, like really belly laugh, they have your best interests at heart, they know your faults but love you for them and accept you for who you are wholeheartedly, they can take the piss out of you with no malice at all and you can take it because you are secure in that knowledge. They are happy for you when things go right and sad for you and try to help you when things are shit.
With my best friend I can tell her anything and I know she won't judge me and vice versa. I think with a real best friend you genuinely love them, mine is like a sister to me and definitely a member of my family. If something happened to her I would be absolutely devastated.
You should be able to be (diplomatically) honest and truthful with a really good friend, and they to you.
My best friend tells me that she loves me every day and me her! Might sound a bit OTT but we cherish our friendship as we have got each other through our twenties and thirties, and are now facing our forties together.
Conversely I have just had to distance myself from another 'friend' as I found she was draining me with her unbelievable narcissism, just going for a drink or walking along the street with her was hard work, telling me everyone was looking at her because she was so beautiful, and how she would only ever go out with a man who was 'really rich' because 'that's what I deserve' and 'feel my muscles in my legs I'm so toned from the gym'.... checking herself out in my sunglasses when i was trying to talk to her, the list is endless.....she was totally absorbed with herself and her image and as a friend I think one can only take so much.... But the worst thing about her that I just couldn't deal with anymore was her blatant 'Schadenfreude' (may have spelt that wrong!) as she absolutely foamed at the mouth with excitement if things weren't going right for me 