Decidueye

Decidueye finds a unique place in the UU tier with its powerful Z-Move and useful secondary Ghost typing giving it some strengths over its main competition in Celebi. It is one of the best dedicated stallbreakers in the metagame, being able to take advantage of many common defensive Pokemon and become immediately threatening after a Swords Dance. Furthermore, its incredibly strong Z-Move lets it push through even Ghost-resistant Pokemon like Bisharp and Alolan Muk after some chip damage or at +2. However, Decidueye's weakness to Pursuit and Sucker Punch from threats like Krookodile and Bisharp forces it to rely on Substitute to avoid dangerous situations with trappers, especially with the prevalence of Dark-types in UU. Their prevalence also hinders it from breaking through non-defensive teams cleanly, though it can still deal some damage in such matchups with its powerful Z-Move. Decidueye's low base 70 Speed also is a huge disadvantage, making it prone to being revenge killed and preventing it from outspeeding many offensive Pokemon like Moltres, Hydreigon, and Rotom-H, needing either a Substitute up or teammates to break these Pokemon. Lastly, Decidueye still faces strong competition from Celebi, which can deal with faster offensive threats significantly more easily because of its base 100 Speed, is more sustainable thanks to Giga Drain, and has a higher Speed tier, more natural bulk, and wider coverage.