Best friend

My neighbour whom I like very much ( as my best friend ) has been asking me this question a lot lately : who's your best friend? I would've told him it was him but he had already told me about HIS best friend who wasn't me so I couldn't tell him the truth. I plan on telling my other neighbour ( a girl ) . Every time he asks me I just say " I don't know let me think " but I really like him and want to tell him. What do I do?