When I think back to Gen 7 OU, Pelipper will always be one of the top 10 to me. It may not have been an A rank mon, but it propped up an entire play style that became relatively common (which imo should count for something) and it was the face of that archetype. All these other mons are good, but not indicative of any one playstyle, so I would not vote for them in this spot as a "Titan of the Metagame"
I'm going for Pheromosa.
The options to stop it are Aegislash, Pex, and Ash Ninja. Aegislash got banned very early, Pex is Koko, Lele and Duggy bait while Ash Ninja needs to have changed form and lock itself into Water Shuriken.
'Mosa was around for a bit but not that long, and while Kartana really only kicked off afte USUM released, it is a defining threat in the tier that has adapted around the metagame and always stayed relevant. Koko is similar, but unlike Kartana in USUM, there have been times where all but one of its sets have been underwhelming.