Kyurem-B is by far the most dangerous pokemon when beeing behind a sub, and with it's great coverage moves and great ability there is really nothing that can beat it, especially in hail as hail negatates Leftovers recovery. Kyu-B also has amazing base Atk and SpA that lets it hit so incredibly hard. And as Shurtugal said having the ability to beat Multiscale and Levitate is incredibly noticable and also helps Kyu-B having more intact subtitutes as without it's ability would have been walled a lot easier. Another cool thing about Kyu-B is that he can have 101 Subtitutes and those are incredibly bulky which are maby the biggest factor for Kyu-B beeing the best Substituter in OU.
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