It was most influential back in good old BW1,a relatively stable metagame deemed to be the homeplace for VoltTurn and BulkyOffense.Back then,HyperOffense wasn't everywhere,and unlike BW2,people weren't used to sacrificing Pokemon left and right and people wanted an actual switch-in on every threat in OU.
Now,there may be some people who may say that Terrakion is not very influential to the metagame as a whole just because it went off-radar in BW2.This way of perception is wrong,and let me explain why.
Terrakion is an "ok" threat in BW2 but was a top-tier threat in BW1.Powerhouses deemed "uncounterable" like Kyu-b,Keldeo,Techniloom,and Thundy-T didn't exist,and these are the pokemon that don't have any real switch-ins.Nowadays we death fodder and revenge kill.Back then they switch in and force out.
But switching in and forcing out isn't the best way to handle Terrakion.High attack,powerful STAB,boosting moves,and an extremely high speed tier in BW1 standards means anything that switches in gets screwed,even if it happens to be Gliscor,Hippowdon,or Skarmory.(Hippowdon is the only one that doesn't get 2HKOed by Choice Band,but back then people didn't realise the usefulness of Hippowdon,just like how Latias,an extremely good pokemon,got neglected in BW1.)
This gave a bit of viability to many lower-tier pokemon,such as Claydol,Golurk,Nidoqueen,and especially Slowbro.Giving viability to lower-tier pokemon is a big testament to how influential it is,or atleast it was,just like how Specially Defensive Ludicolo,Gastrodon,and even Shedninja have little viability in Ubers just because they can counter the most infuential pokemon ever:Kyogre.
These days,almost all pokemon deemed "uncounterable" like the ones listed above are just revenge killed after death foddering something.Same goes for Terrakion.Since now everything is handled that way,including Terrakion,Terrakion fails to stand out amongst the crowd and so,drops in usage.
Since Terrakion was super influential in BW1,almost reaching Kyogre levels,it mitigates the fact that it was not so meta-defining in BW2.
And that folks,is why I am voting for Terrakion.