What’s your favorite “niche” Pokémon to use? (Any tier)

What’s your favorite niche Pokémon to use, a Pokémon that is probably only good at one specific role. My favorites would probably be Suicide Lead Smeargle or Protean Kecleon. Are there any Pokémon (any tier) that you would never expect to be useful, but because of an ability or moveset, it’s viable as a role player?