Sponta, its not nesseary that he/she be someone who spend most of ur time with or someone who you know for many years...etc. Its someone who you trust & enjoy his/her company with. o if he/she stab's you in the back like miya said.. then he/she ain't a best friend after all!
if someone calls me at 3 am to talk just because i am supposed to be a "best friend" i'll hang up on them !! seriously people taking this way too far , they are over reacting. because that friend is a person with needs too , they need to sleep and have their own time "alone" ... to me a best friend is someone whom i enjoy a good laugh with and never nags on my head !... w amaze me with his/her thoughts bs ;-p