Zygarde
I really don't know why people are saying landorus-t. The z sets are undoubtably strong and its other sets are very effective, but it is nowhere near as metawarping as all the other things Intoduced this generation. Lando is a PRODUCT of the meta; not a defining force -- just strong, versatile, and consistently does its job. If you find yourself in a meta where 10pokemon can threaten to sweep of a boost, or even just a kill, you bet I'm running something that can take a hit with intimidate.
Zygarde forced nearly everysingle team to take a grass type, and would practically autowin if they didn't.
Beyond zygarde there's also a bunch of other stuff that forces so much speed control this generation, and is a big reason why a lot of scarfers rose to prominence: (Ash) Greninja, Koko, scarf Lele, Hawlucha, kartana...and the banned ultra beasts. Or the new walls that require specific STAB breakers like toxapex or magearna.
(Real answer is Nagandel for requiring use of heatran or Marsahdow for being broke, but they werent in OU for long enough)