Angel Batista would have to be considered Dexter's best friend.
He doesn't know about Dexter's killing habit, of course, but Angel is still the one guy Dexter can sort of talk to and bond with.
Angel's personal life is a bit of a mess: he's divorced from his wife and doesn't see his daughter very often. He also got way too involved with that nut job Lyla in season two.
Still, Angel seems to be a genuinely well-meaning, kind-hearted detective. In season three, he was promoted to Sergeant. On season four, he actually married LaGuerta.