Zygarde
I see what you mean, but in a lot of instances the broken Pokemon do warp the metagame around themselves for as long as they're there. Zygarde at least wasn't horrible like Keldeo or Diggersby where they had zero counterplay (well, to be fair Latias was UU the same time Keldeo was) and you just had to bend over and hope it didn't last too long. Zygarde did pretty effectively break the bulky water metagame, forcing basically all of them to mess with their movesets and run Ice Beam. Honestly, if Milotic had been popular then before Zygarde dropped I could have seen it staying a lot longer than it did, since it always had the space for Ice Beam.Florges
Zygarde was more broken than influential imo this is becoming more like a list of most broken Pokemon to grace UU and not the most influential