Kind of hard to say, as a metagame is an interwoven and complex beast. In terms of averaging large quantities of stats, everything is chained together. Gliscor affects Terrakion usage, Terrakion usage affects Heatran usage, Heatran usage affects Scizor usage, etc.
Shooting for the obvious, I'd say that the Pokemon which have the largest impacts are those which enable the success of entire playstyles. This would be Politoed, Tyranitar and Ninetales. Although handling each of these Pokemon individually is a small task, teams require multiple checks to the entire weather teams that these Pokemon are responsible for due to their abilities. The very roots of the 5th generation metagame are grounded in controlling weather. Deoxys-S was also influential in this regard before it was kicked out of OU, as it enabled the success of hyper offense (and as we have seen, the game has drastically slowed its pace since the ban).
Some guys like Scizor keep the usage of a lot of Pokemon up (Magnezone) or down (Latios) and may even affect tiering (if Scizor wasn't around, would Dragonite be uber, etc). But I don't think it comes close to the impact that weather starters have, really.