Core: Registeel / Slowbro

The classic RegiBro core returns in BW with a number of useful new features. Registeel remains much the same as before, but Slowbro received an outstanding new ability in Regenerator and two solid STAB moves: Scald and Psyshock. Other than that, the two form a core that can easily take on some of the most dangerous physical and special threats in UU.

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Registeel @ Leftovers 

Trait: Clear Body 

EVs: 252 HP / 4 Def / 252 SDef 

Careful Nature (+SDef, -SAtk) 

- Stealth Rock 

- Iron Head 

- Seismic Toss 

- Toxic

Slowbro @ Leftovers 

Trait: Regenerator 

EVs: 252 HP / 252 Def / 4 SDef 

Bold Nature (+Def, -Atk) 

- Calm Mind 

- Scald 

- Psyshock 

- Slack Off

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Registeel covers most special threats with its gigantic Special Defense stat. It also works as a reliable Stealth Rock user and status spreader, thanks to its bulk and natural utility. Iron Head provides Registeel with a solid STAB move to hit Ghosts (notably Mismagius) with, while Seismic Toss is a great all-around move on a special wall. Toxic is preferred over Thunder Wave for hitting Ground-types and bulkier opponents.

Slowbro patches up its partner's weaknesses by functioning as the best physical wall in the tier. With a Calm Mind set, it can also take over as a bulky setup sweeper. Scald works nicely as a solid STAB move with a 30% burn rate, capable of crippling physical sweepers. Psyshock hits common switch-ins on their physical side, almost guaranteeing a OHKO after Stealth Rock on max HP Roserade. It also lets Slowbro win one-on-one against many other Calm Mind users. Slack Off rounds out the set by giving Slowbro reliable recovery, although Regenerator's 33% heal rate keeps Slowbro healthy just by having it switch out. 

These two Pokémon have some of the best defensive synergy in the game, making them the UU equivalent of SkarmBliss. Pair this core up with hazards, bulky offensive members and a late-game cleaner, and you'll have a solid UU team on your hands.