Crustle (Yes, I'm being incredibly specific here for a reason)
The Crustle you can catch in the Seaside Cave is no use against Marlon for obvious reasons, but just you watch as he ploughs through Team Plasma's Frigate all on his own (this is precisely what I ended up doing). Just one level off Shell Smash when you capture it (easily remedied with a Rare Candy) and equipped with Sturdy, you can almost guarantee a sweep. Hell, sometimes you can manage two Shell Smashes and almost everything will fall to a Rock Slide, X-Scissor or Bulldoze. Caitlin got absolutely crushed by Crustle. Oh, and so did Black Kyurem. Couldn't KO me with its Fusion Bolt, got off Shell Smash, managed survive another hit and then BAM, dead Kyurem. Didn't go nearly so well vs Ghetsis, mind.
One thing that sucks about Crustle is his speed. 45 speed is slow. With one Shell Smash you bring that up to 90, which can put you past most of the weak stuff the Plasma Grunts bring out (and the majority of Caitlin's team) but against faster opponents you could well be setting yourself up for nothing but disappointment...
Still, I'd say High Tier simply based on how destructive Crustle can be if used properly.