Celebi

Celebi's main niche in BW OU is as a bulky pivot due to its good resistances, bulk, and utility movepool. Celebi is especially effective against rain teams, as it walls Keldeo unless it runs the otherwise inferior Hidden Power Bug while also being able to shrug off attacks from Latios and Thundurus-T. Celebi is also unique in that it doesn't mind Scald burns due to Natural Cure, letting it take strong hits, heal, and U-turn out to gain momentum. Additionally, Celebi is probably the best Breloom answer in the game, as it takes even Swords Dance-boosted attacks well. Celebi also helps shore up teams against Alakazam and Reuniclus lacking Signal Beam, as it is able to safely pivot in a Pursuit user like Tyranitar with U-turn. Celebi has a huge support movepool and can run options such as Nasty Plot, Thunder Wave, Worry Seed, Stealth Rock, and Perish Song. However, Celebi is uncommon because of several crippling flaws, primarily its propensity to getting forced out by common Pokemon like Volcarona and Heatran, weakness to common moves like U-turn and Ice Beam, susceptibility to being worn down by Spikes, and vulnerability to Pursuit. Although Celebi walls most Pokemon on rain teams, Ferrothorn walls it and can use it to set up entry hazards while pressuring it with Knock Off. This issue, in tandem with its vulnerability to entry hazards, necessitates Rapid Spin support, making the team structures Celebi fits on quite limited. Being a Stealth Rock user so weak to Volcarona is also a huge vulnerability, as it can come in easily and start setting up without having to take Stealth Rock damage.