Pirog is wasting away his talent by not fighting anyone. When he does fight he puts up uninspiring performances like he did against Ishida. He's probably the most skilled MW. But again he continues to rot away.
Golovkin has a lot of potential. I believe when he KO's Gabe Rosado he should fight Danny Geale for the title.
I like Geale a lot, he's legit.
Peter Quillin, he's a bit overrated. He dropped N'Jikam 6 times and couldn't finish him. That usually means he doesn't have good boxing ability. He's explosive, has power, but he doesn't really have the skills to beat guys like Golovkin or Pirog, and definitely he can't beat Sergio.
I think it's going to be either Sergio, or Danny Geale and Golovkin if they fight each other.