Registeel

Registeel has amazing bulk and a great pure Steel typing, which together allow it to check Dragon-types such as Latias and Hydreigon and Flying-types such as Togekiss and Crobat. It can also support its team in various ways by setting Stealth Rock to damage foes as they switch in and by crippling opposing Pokemon with Toxic. However, Registeel is very passive, relying on moves like Toxic and Seismic Toss to do any significant damage to foes, which means that it can be easily taken advantage of and thus only fits on defensive teams. Registeel also faces competition from other Steel-types like Mega Aggron and Mega Steelix, which are way less passive than it. Unlike them, however, Registeel can hold Leftovers and, more importantly, doesn't take up a Mega slot, meaning there's a lot less opportunity cost when using it. Registeel also sports a higher Special Defense stat than both Mega Aggron and Mega Steelix, letting it more easily switch into Pokemon like Primarina. It also faces competition from Bronzong, but having access to Counter and not being weak to Pursuit gives it a slight edge over Bronzong. Furthermore, Registeel's weakness to common typings such as Ground, Fighting, and Fire also hold it back, as it can be easily exploited by Pokemon like Nidoking, Cobalion, and Infernape. It also lacks reliable recovery and is forced to rely on Leftovers to remain healthy, so it can be worn down rather quickly.