Serperior

When it comes to late-game cleaners, Serperior does its job very well thanks to its superior Speed tier and surprisingly solid natural bulk. With said Speed tier, it's able to outspeed and heavily damage many threats in the metagame like Rotom-S, Charizard, and Haunter. It can run a Calm Mind + Substitute set to become a more formidable sweeper. Hidden Power Rock and Ground round out Serperior's coverage quite nicely, hitting Flying- and Poison-types, respectively. Hidden Power Ice can be used to hit Dragon- and Grass-types; however, thet are fairly uncommon and typically not worth the moveslot. Despite all this, Serperior usually needs at least a single Calm Mind to be an effective sweeper due to its subpar Special Attack stat. While it is naturally fast, there are many faster threats that offensively pressure it like Choice Scarf Rotom-F, Swellow, and Floatzel. There's also a multitude of special walls like Miltank, Roselia, and Zweilous to boot that simply laugh at Serperior's attempts to take them down.