I don't think Luffy is the kind of guy to have favourites, but regarding who he is closest to I would say Zoro. Not because they have been together the longest, but because they understand eachother. They both have a goal they want to achieve no matter what and they both understand the way of the warrior.

@Assertn: I really can't see were you got that assumption from, there's nothing here that implies that Luffy doesn't trust any of his mates.