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.
Angel: You know, Pasquale is just caught in a cycle of reaction. She's not co-creating her own reality.
Dexter: You know those words don't actually mean anything, right?
Angel Batista: [about the body found on the ice of a hockey arena] What do you think he's trying to tell us?
Dexter Morgan: That hockey's a violent game?