I just want to point out that "staple GOOD Pokemon" such as Garchomp, Terrakion, Keldeo, Genosect, Darkrai, and Landorus-I (pre-ban) have a major impact on the game, even if it's not often at the forefront of people's mind (I'd say Jirachi also kind of fits this bill starting in BW2). Pokemon that really, have it all-- typing, stats, ability, powerful/good moves, and good abilities. All they lack is some stat, some function, some capability or quality that breaks the game-- keeps them from being an Excadrill, Deo-S, Shaymin-S or Swift Swim Kingdra (Garchomp's SS sets were found to eventually have had this quality). Often though, these Pokemon are much more independent (of need for support), and perform more consistently than their more broken counterparts, who often rely on weather or some other condition or luck to show their truly broken colors. This consistency of performance, this reliability, is what makes "staple Pokemon" so important to the metagame-- SHAPING the metagame.
A set-up Blaziken, Venusaur, Kingdra, Gyarados, or Tornadus-T may be more devastating. The support from a Darkrai sleep or a Deo-S may have the potential to break the opposing team in half-- and you might be 6-0'd by Shaymin-S should it get lucky. Bringing a Dugtrio might let you kill many a Ninetales, win many a weather war, and pull off many a win. But, there is ALWAYS a risk to building too heavily on a strategy, and relying too heavily on risky and "abusive" strategies-- because you worry if your high risk members can consistently out compete and out perform Pokemon that are FUNDAMENTALLY good-- staple Pokemon. These Staple Pokes, that almost always perform consistently; always forcing you to weigh the risk.
No matter the era, no matter the metagame-- every time you put a Kyurem-B, Venusaur, or Excadrill on your team you have to ask yourself, "why am I using this over [Garchomp, Terrakion, Keldeo, Genosect, Lando-I, etc.]". Even when you KNOW Excadrill's Speed decides many a game, there's that nagging voice in the back wanting to hedge bets, wanting to choose the much less risky, much more independent Garchomp. At the very least, you try to include a number of Staples alongside high risk performers-- to support, and hedge bets. It's in this way that Staple Pokemon shape the metagame, putting massive influence despite not being its most obvious element (like weather or hazards).
Consistently performing Staple Pokemon shape the metagame-- because they are the standard to which ALL OTHER Pokemon must be judged. You can say that Staple Pokemon define the metagame by drawing the base line that judges whether other Pokemon are good or not-- whether they deserve use in OU or not. Of these Staples, Genosect is the absolute best-- the most powerful, the most consistent. But... not the most influential.
Out of these, I'd have to say the most influential is Garchomp. Really, Garchomp is the first of its kind to even exist-- the first Pokemon to have such a flawless typing, movepool, and stat spread that you constantly ask yourself why you're NOT using it. Sure, in BW all these others came to being as well-- but Garchomp was also the standard to which ALL OF THEM were judged. Is Terrakion's Stone Edge strong enough, do its attacks hit hard enough, is that base 6 Speed important enough to excuse its extra weaknesses (and reliance on Stone-miss) compared to Garchomp? Is rain's boost and mixed attacking good enough to excuse the hole in Keldeo's STAB coverage compared to Garchomp? Is Genosect's versatile movepool and STAB U-Turn enough to excuse the poor Speed in a Speed metagame and Choice Scarf competition that was DEFINED by Garchomp.
Even Pokemon like Excadrill and Deo-S found themselves on the wrong side of broken... in large part because they could too easily out speed CS Garchomp.
Staple Pokemon define the meta-- they have a profound impact on its shape. Out of these, none has shaped the meta, and shaped people's expectations-- even shaping the way we judge other staple Pokemon-- more than Garchomp.
If we're talking about "influence", I think enough can be said here. Garchomp is the gear at the center of the OU wheel, the "mark zero" of Pokemon power rankings in OU. Pokemon that are obviously weaker than it are almost always decidedly OU-- not broken. Pokemon that are obviously better than it find themselves under severe scrutiny-- even when they are not broken (see: Keldeo). Pokemon that are weaker/less consistent than it must have some capability that makes them more dangerous under the right conditions (see: Dragonite, Volcarona, Gyarados). Even defensive Pokemon have to be at least as bulky as Garchomp, or have more resistances, in order to truly be a good OU wall. It is the Pokemon by which all other Pokemon are judged. Even banned it had this position, and its importance is clear in our effort to bring it back.
By having those incredible stats, incredible typing, incredible movepool-- but STILL not being "broken", Garchomp has become a central cog of the metagame that shapes our expectations. As a central cog, Garchomp really pushes the envelope on OU's power standard. Without Garchomp as that standard-- let's just imagine if a Pokemon like Gyarados was the standard instead-- I think we'd be looking at the formation of an overall much weaker OU.
Inversely, I guess it was made pretty clear that people didn't want a metagame where Genosect was the central cog to which others would be judged.
As a staple, Garchomp has done so much, and influenced us so much, in building the OU we have today.