Registeel

While Registeel is often overlooked in favor of other bulky Steel-types, such as Mega Aggron, which has a much greater offensive presence and access to Filter, and Bronzong, which has Levitate and takes neutral damage from Fighting-type moves, it still has several things going for it that make it a viable option. Registeel boasts incredible 80 / 150 / 150 bulk and a pure Steel typing that grants it important resistances to Dragon, Fairy, Rock, Flying, and Grass, as well as an immunity to Poison. These attributes make it a great check to some of UU's most powerful threats, such as Hydreigon, Venomoth, Kyurem, Mega Aerodactyl, Mega Beedrill, Shaymin, Crobat, and Porygon-Z. Registeel is also a very reliable Stealth Rock user and, unlike Mega Aggron, can hold Leftovers, has a phenomenal Special Defense stat, and doesn't take up your team's Mega slot. However, Registeel has little offensive capabilities due to its low Attack stat, unless it uses Curse. Therefore, it should only be used on very defensive or stall teams. No access to instant recovery also means that Registeel is prone to being worn down.