Celebi

Celebi is one of the most versatile Pokemon in ADV OU. With high stats across the board, it can easily run physically and specially defensive sets, momentum-maintaining Baton Pass sets with Calm Mind or Swords Dance, a special sweeper set, or a Substitute + Baton Pass set.

Defensively, Natural Cure and crucial resistances to Electric-, Water-, Ground- and Fighting-type attacks allow it to pivot safely into Zapdos, Jolteon, Magneton, Swampert, defensive Suicune, Claydol, and Breloom without Hidden Power Bug and into status moves from Blissey, Snorlax, Defensive Jirachi, Milotic, and Porygon2. Leech Seed is the best move defensive Celebi can offer, allowing it to not only soften the impact of incoming threats but also chip bulky Pokemon into KO range of its teammates.

Offensively, it is one of the few pivots that can easily turn the tides against Zapdos and Swampert by absorbing Thunderbolt or Hydro Pump, setting up, and Baton Passing out; it is perhaps the only common and reliable stat boost passer in the game. Celebi as a sweeper itself is often also very dangerous in an unconventional way, because its coverage and natural bulk allow it to challenge checks to more passive sets like Tyranitar, Salamence, and Metagross, while its ability to heal from Giga Drain may very well help to win the war against Snorlax and Blissey.

Unfortunately, while Celebi's typing offers all its useful resistances, it is also the root of all its flaws. Weaknesses to very common attacking types like Flying, Fire, and Bug invite all sorts of threats like Salamence, Moltres, and Heracross, while its need to repeatedly use Recover in the face of Ice Beam to gain back a substantial amount of HP makes it very susceptible to freeze and critical hits. Its four-fold Bug weakness makes it susceptible to being trapped by Dugtrio, and its Dark weakness makes Pursuit Tyranitar a huge threat to any Celebi without Baton Pass. While Celebi can often be considered a special wall, its inability to pressure Skarmory and weaker special bulk make it relatively weaker to Spikes and sand than Blissey.