I think there is a real possibility that Masuka is the skinner.
Firstly his motivation could be that he's pissed at everyone else and wants to prove how much smarter he is than anyone else.
Secondly, I think he knew from the start that Ramones victim wasn't a skinner victim, because he did it himself.
Also, in the books, Dexter thinks that Masuka is also "pretending to be a human". He notices that Masuka has poor social skills and is awkward and he believes this is because he is also harbouring a dark passenger.
The only downside to this is that he would be kind of lame if another member of the miami metro is a serial killer.
Miguel also has alot of reasons that he could be the killer. In the next episode sneak peek we see Miguel is trying to convince dexter that the defense attorney is worthy of being taken out. So perhaps it is Miguel who has found someone like him, and is first only letting dexter take out guilty people, hoping he gets an appetite for murder and will join him in outright murders.
If its Quinn they better provide an interesting reason for it being him, rather than it just being him.