My brother is one of my best friends. We have similar interests, politics, taste in music, taste in people - lots of my friends are now his friends too, and vice versa. His wife is also one of my best friends and they are my DC's legal guardians.
I think this is partly luck and partly the fact that we lost our dad in our late teens, which was a lot to go through and brought us closer together.
I'm not sure if I actually prefer him over all my friends, though, especially since I think there's something irreplaceable about female friendship that I obviously can't get from my brother.