Greninja

Water

Greninja is the most versatile Water-type thanks to Protean giving it STAB on all of its attacks, enabling it to use almost any attack for coverage and still do great damage. It has an amazing movepool to maximize use of Protean including Ice Beam, Gunk Shot, Extrasensory, and many other coverage moves, which makes it a fantastic wallbreaker if it has the appropriate super effective attack. Expert Belt allows Greninja to take advantage of that fantastic coverage and ability Protean, allowing it to take on Flying, Water, and Ground teams quite easily. Greninja's amazing Speed makes it hard to revenge kill when it uses a Choice Scarf, able to outspeed almost the entire metagame, and allows Greninja to act as a great offensive pivot. In a completely different capacity, Greninja is also one of the few reliable ways to beat Poison teams for Water with its Groundium Z set. As a result of its many sets, Greninja is highly unpredictable and fits on almost every Water team, being able to fill any role the team needs. However, other than its Speed, Greninja has awful base stats, with almost no bulk to speak of and poor offensive stats. Furthermore, while its Speed is its only good stat, it still falls short of Tapu Koko, which is one of the biggest threats to Water teams, and can be easily revenge killed if it lacks Choice Scarf when considering its awful bulk. Greninja relies heavily on super effective damage to patch up its offensive stats, making it struggle to choose four moves from its huge movepool. Greninja will simply always be walled by something due to this restriction.

Dark

Greninja is one of the best offensive Pokemon that Dark teams have access to thanks to its powerful ability in Protean and to its diverse coverage options such as Gunk Shot, Ice Beam, U-turn, and Extrasensory making it an unpredictable threat. Protean also comes in handy defensively; for example, Greninja can become Psychic-type by using Extrasensory to avoid getting KOed by Keldeo's Secret Sword. Greninja sits at a splendid Speed tier, outspeeding common threats such as Garchomp, Mega Diancie, and non-Choice Scarf Keldeo. Greninja's use of an Expert Belt is complemented by its ability Protean, making it painful for many teams to deal with including Flying, Ground, and Water teams. When equipped with a Choice Scarf, Greninja functions as a Dark team's revenge killer, outspeeding threats such as Mega Lopunny, Tapu Koko, and other Choice Scarf users such as Tapu Bulu and Latios. Greninja has exploitable defensive stats, however, making it unable to take several hits and easy to revenge kill with priority attacks and faster attackers. Although Protean boosts attacks, Greninja is still held back by its average offensive stats, which means it's reliant on Life Orb to further boost its damage output. Greninja also lacks the room in its moveset to check every threat Dark teams might face.