Carracosta

Carracosta is a powerful Pokemon thanks to its combination of Shell Smash, mixed offense movepool, Sturdy, and priority moves. With Shell Smash and Waterium Z, it can outpace and OHKO a lot of Pokemon such as Heatran, Landorus-T, and Porygon-Z, while priority in Aqua Jet allows it to beat opposing Sturdy Pokemon such as Magnezone, Crustle, and Trace users like Mega Gardevoir. Because Carracosta can run a mixed set, it can easily threaten defensive Pokemon and isn’t threatened by Will-O-Wisp from Mega Charizard X or Mega Gardevoir like many other Sturdy Pokemon are. Unfortunately, Carracosta can't effectively run mixed STAB attacks, priority, Shell Smash, and coverage in one set and individual sets lose out on key power that could be helpful. It also is reliant on its STAB moves, meaning Pokemon that resist them like Mega Venusaur can easily beat it. It's also extremely slow and can be beaten by faster priority users like Donphan or can be prevented from setting up by Taunt users like Tapu Lele.