They were, then I think Sheldon and Penny became best friends, but at the moment, especially after last week's ep, I think Leonard/Sheldon are best friends again.

I do think they're best friends, maybe not completley in the traditional sense all the time but, I think they have each others back when it comes down to it.

I think it's quite possible to have two best friends especially if one is a guy and the other is a girl, there are still some boundries between genders even with best friends.