Greninja

Greninja might seem like an unassuming, mediocre Pokemon on the surface, but just like a real ninja, it's hiding a deadly weapon under its unassuming appearance in the formidable Protean! Protean gives Greninja STAB on every one of its attacks, making it deceptively powerful, and Greninja's high Speed and diverse attacking movepool let it revenge kill dangerous threats such as Garchomp and punish most offensive switch-ins. However, Greninja isn't very versatile on the item front; it's forced to use either Life Orb or Focus Sash, restricting the item options of your team. Greninja's defenses are rather subpar, and its typing will usually only protect it from Sucker Punch and situational attacks via Protean typing changes. Greninja also has a difficult time covering the whole metagame with just four moveslots, and with the moves it has, Greninja will often just miss out on a KO against healthy Pokemon. All of these aspects combined mean that Greninja must be played with care, or else it will simply waste a Pokemon and item slot. Greninja might not be the easiest Pokemon to use, but don't let your guard down, because Greninja is a Pokemon that is worth more than the sum of its parts. Protean is still an excellent ability that gives out game-changing rewards with good prediction, thanks to its type-changing properties giving Greninja the chance to survive moves that would normally OHKO it. This means that a well-played Greninja is among the most deadly assassins in the Battle Spot Singles metagame, so giving a Greninja a contract for your team is a fine choice as long as you're prepared for its flaws.