Ok, time for my list;
1. Politoed
This should be obvious. How many bans have there been to nerf rain? There has been Swift Swim, Manaphy, Thundurus-I (though it is debatable whether this one was due to rain) and even Tornadus-T, who I don't think is even broken. Even with all these bans, rain is still one of the most popular playstyles in OU. Another important thing to note is that rain has been dominant throughout BW1 and BW2, not many pokemon can claim to have warped the metagame so much over such a long period of time. Drizzle is the most metagame-defining force in OU, period. This makes Politoed the most metagame-defining pokemon in OU.
2. Tyranitar
Another weather summoner, Tyranitar has the advantage of being a very useful pokemon outside of just summoning sand. He is the best user of Pursuit in OU, easily trapping threatening pokemon like Latios. With the boost to its SDef courtesy of the sand he summons, he is also viable as a special tank (I won't say wall as it lacks recovery outside Leftovers) as it has the equivalent of base 154 SDef, that is the same as insta-Uber pokemon like Ho-Oh. The sand it summons is another driving force in the metagame, though the only ban was that of Excadrill. I would go so far to say that without Tyranitar, Politoed (or probably just Drizzle) would have been insta-banned. Tyranitar has always been metagame-defining, and GenV is no exception.
3. Ferrothorn
Ferrothorn is an anomaly in BW, it has avoided the trend of overpowering threats taking over OU and has made a name for itself as a mixed wall and Spikes setter. It has been a staple throughout BW1 and BW2, doing its job remarkably well to the point of replacing former stalwarts like Skarmory and especially Forretress almost completely. Its advantages over those two are its much higher SDef and typing suited to taking common attacks on that side. Ferrothorn fits well into almost any type of team, it is immune to sandstorm and rain halves its biggest weakness, making it even more formidable. Ferrothorn is one of the main reasons that pokemon like Dragonite and Latios are still allowed in OU as it can take +1 Outrage and Choice Specs Draco Meteor like they are nothing. Its presence is also as much of a hindrance to rain as it is a blessing, it practically counters rain by itself apart from Keldeo and the rare Toxicroak. A new face with a big impact, Ferrothorn very much deserves this spot.
4. Garchomp
Of all the dragons, Garchomp is the most metagame-defining. It sets itself apart from the pack in a variety of ways, so much so that it was Uber for the majority of BW1. For a start, it has incredible stats. Its fast, bulky and powerful, with enough SAtk to make Fire Blast viable to pick off the steels that don't fear Earthquake. This brings me onto the 1st real defining factor; its typing. It gets STAB on Earthquake, making it much more than just a coverage move, it is a wallbreaking/sweeping tool in its own right. Also, it grants a resistance to Stealth Rock as opposed to a weakness that flying dragons have. Next, its speed trolls all the base 100s, along with Landorus-I and Thundurus-T. Garchomp has an extensive movepool that allows it to fill many roles on a team; it can be a wallbreaker with Choice Band or Swords Dance, it can be a sweeper with SubSD, it can be a dedicated revenge killer and cleaner with Choice Scarf, it can even be a good Stealth Rock lead or defensive tank. The final thing that pushed Garchomp over the edge was its ability; Sand Veil. Now however, Rough Skin is still very useful. All the dragons shape the metagame, but Garchomp is the only one to go all the way over the edge, even inspiring a change in the Evasion Clause just to bring him back down from Ubers. This cements its slot on this list.
5. Ninetails
The last of the main three weather summoners; Ninetails would be in NU if it wasn't for its ability. However, Drought is essential for sun teams to compete in OU, so Ninetails is a necessary evil for them to have. Chlorophyll is still allowed in OU, Venusaur is the best example of a Chlorophyll abuser that has made a significant mark on the OU metagame. Volcarona, though not reliant on sun, is easily the best fire type sweeper in OU and is very common on sun teams. Regardless of Ninetails worthlessness on any team that isn't based around sun, sun teams are extremely potent and are a force on the metagame, so Ninetails is as metagame-defining as those teams are.
6. Terrakion
In my humble opinion, Terrakion is the most effective physical attacker in OU right now. It is so difficult to handle, it only has a select few checks. It shares many of its qualities with Garchomp; an incredible STAB combination, a trolly speed tier, and vicious stats. Its attacks are much less limiting than those of Garchomp, Close Combat allows Terrakion to switch out as opposed to Outrage, while Stone Edge doesn't have any immunities and hits key targets, despite its low accuracy. The other two slots are much more flexible, allowing it to fill many roles on a team. It can go for specific coverage moves on Choice sets (and even Quick attack on Choice Band) or different types of set up moves. Taunt and Stealth Rock turn Terrakion into a very effective lead while SubSD allows it to punch through even the likes of Landorus-T and turns it into a sweeper that breaks walls for itself. Anybody that builds a team without one of the very few Terrakion checks will be in for a world of pain, it forces people to use Landorus-T or priority like Mach Punch or Bullet Punch. This cements it as a metagame-defining force and elevates it to the slot it deserves on this list.
7. Landorus
Since the release of Sheer Force, Landorus has become one of the most feared sweepers in OU. The strange array of moves that Landorus is forced to use somehow pull together to give extremely good coverage, allowing it to sweep teams clean after a Rock Polish. The counters to special Landorus all tend to be weak to Pursuit (think Latias, Celebi and the blobs) so Tyranitar is a good partner. The physical sets also like having Tyranitar for a partner as they rely on Sand Force to boost their power. The special set is what makes Landorus so defining though, it forces the use of at least one of its counters.
8. Keldeo
Since its release at the start of BW2, Keldeo has come into its own as a force in OU. It was first used almost exclusively as a Choice Specs wallbreaker in rain, Hydro Pump is so powerful that it OHKOes Dragonite after Stealth Rock while Secret Sword bypasses special walls like Ferrothorn and both blobs. However, more sets have arisen, such as Choice Scarf and Calm Mind sets, each of which do their jobs admirably well. Choice Specs Keldeo fits in best on rain teams as the boost in power allows it to do incredible things, but Choice Scarf Keldeo fits best onto sand teams as most of its checks are weak to Pursuit, meaning Tyranitar can very easily pick them off. Keldeo is like a special version of Terrakion, except it relies on the sheer power of water moves instead of the combination of two incredible STAB moves. It has a specific set of checks, any team without them is doomed to fall at the hands (hooves?) of Keldeo, warping the metagame and making it a staple on this list.
9. Rotom-W
Rotom-W is one of those pokemon that does so much for a team that it is hard to overlook. It does well at taking on rain teams, only really fearing Ferrothorn and Secret Sword Keldeo. And other rare threats I suppose like Gastrodon. It also does decently against sand, not many common members of sand teams like taking on Rotom-W. This alone warps the metagame a bit as these teams must cater for this weakness. The real reason that Rotom-W warps the metagame is that it is a staple on VoltTurn teams. Commonly partnered with Scizor, and Genesect when it was OU, VoltTurn tore up the metagame to the point that questions were raised about whether it should be banned. Now, VoltTurn isn't very common any more, but Rotom-W is, and it is a pokemon that can't be forgotten when building a team, making it a well-deserved member of this list.
10. Heatran
Heatran is similar to Ferrothorn in that it is an anomaly. With the dominance of rain, how on Earth has Heatran managed to stay in the top 5 most used pokemon month after month? Well, Heatran isn't completely useless against rain, it can absorb Hurricane and set up Stealth Rock on Ferrothorn or Jirachi. And against other teams, Heatran is extremely useful. For a start, it walls 90% of the pokemon used on sun teams, this is incredible and is one of the main reasons Heatran has stayed afloat in OU. Also, Heatran is a good check to dragons if it can avoid Earthquake and Surf from Latios, it is even immune to the fire moves that take out other steels. Its typing is unique, and helps it with walling certain pokemon that can cover other steels with a fire move. Heatran isn't limited to defensive sets either, there are a vast majority of (admittedly rare) offensive sets around, even trapper sets become viable to help Ninetails outlast the other weather starters and ensure that sun stays up. Heatran is incredibly useful in OU, meaning that team builders must always keep it in mind. Despite the dominance of rain, Heatran secures the last place on the list of metagame-defining pokemon.
That was my list, with opinions on why they are so defining. Note that pokemon like Skymin, Blaziken and Genesect aren't on there as they don't affect the metagame the way it is today seen as they are banned to Ubers. This is my list of the pokemon that are having a continued effect on the metagame today. Other pokemon that I considered for the list are Scizor, Latios and Jirachi but I ultimately decided on the 10 above.