Mew

Mew's niche comes from its typing, stats, and movepool, which allow it to defensively check a few top-tier Pokemon such as Landorus-T, Mega Medicham, Garchomp, and Gliscor and provide Stealth Rock or Defog support. It's also a terrifying sweeper, as it can easily find setup opportunities thanks to its good bulk and typing, and its wide movepool enables it to pick its own checks and counters. However, status ailments are prominent in the metagame, and Mew wants to stay away from them, as they compromise its ability to check threats such as Mega Medicham reliably if hazards are up, which may very well be possible if Mew is used as a Defogger. Furthermore, defensive Mew's rather passive nature can allow Pokemon such as Ash-Greninja and Mega Alakazam to switch in freely, depending on the moves it is running.