Quagsire

While Quagsire faces a lot of competition from its rival Water / Ground type Gastrodon due to the latter's better Special Attack and special bulk, it does have a unique niche in Unaware, an excellent ability that lets it ignore boosts from setup sweepers entirely. This lets it effectively wall the likes of Swords Dance Scyther, dual dance Rhydon, and Malamar, making it a very helpful asset for stall teams. While Gastrodon can use Clear Smog to remove boosts, this does not work against Steel-types such as Klinklang and it doesn't let Gastrodon ignore boosts as it switches in or until after the move executes, making Quagsire much more reliable in this regard. Regardless, Quagsire must watch out for powerful special attackers such as Jynx and Samurott due to its lower Special Defense stat, as they can deal heavy damage with unboosted attacks. Grass-type Pokemon also deal tremendous damage to Quagsire, and most fear only status when switching into it. While Unaware is certainly a great ability, you should decide whether it's necessary for your team to function well before choosing Quagsire over Gastrodon.