Depends... if either of you has wanted to date the other, or if you have dated in the past... I don't think you can be best friends without something awkward happening.
I've been really good friends with girls (back in my dating days), but I'm not sure they would have been called my best friend. Probably my best female friend that wasn't my girlfriend...
I dunno, I think it just boils down to the whole sex thing... and like I said, if there's a history there or if either of you would ultimately like to get sweaty and naked... the best friend thing will fail.
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