A guy maybe is your best friend if you ACCEPT him as your best friend without expecting from him something in return. There is really no definete answers from this question EXCEPT when you considered some qualities from him in your favor. For me there is only a BOSSUM FRIEND, INTIMATE FRIEND, CASUAL FRIEND but when you say BEST FRIEND i think it is the combination of these qualities and traits that every person is seeking and equate this into his own understanding and likeness but that is very very rare to find in one personality. Literally you can call you most helful or favorite friend a BESTFRIEND but in the true sense of the word i think it is not.