The OP is obviously outdated so I'll give my take on how the VR should look and hopefully spark some discussion. The specific subgroup divisions are hard to narrow down so I'll just list the mons ordered by viability.
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1.
Tyranitar
2.
Ferrothorn
3.
Latios
4.
Landorus-Therian
5.
Excadrill
6.
Garchomp
7.
Keldeo
8.
Politoed
9.
Heatran
10.
Gliscor
11.
Tentacruel
12.
Rotom-Wash
13.
Breloom
14.
Reuniclus
15.
Amoonguss
16.
Magnezone
17.
Thundurus-Therian
18.
Alakazam
19.
Jirachi
20.
Skarmory
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Tyranitar starts off this list as the most defining mon in BW OU. Weather is very central to the metagame and Tar is the most consistent setter. Sand not only negates rain/sun but also chips lots of mons with residual beyond that. Even without the weather war aspect, Tar is still extremely useful as a trapper, whether that's with chople or a scarf. I disagree with
dice's "sr tar is trash" analysis because this is very easy to alleviate by running dual sr sand, and this can be done with something like Ferro or scarf Lando. I also think Sand is far and away the most consistent weather, with even rain being very far behind in terms of consistency, as it relies way more heavily on rain being up than sand does on sand being up (think of mons like tenta and the genies).
Ferro is next because it's the premier spiker and can fit on literally any sand or rain team. Spikes, sr, whip, gyro, leech, knock, twave, protect, and worry seed are all valid utility options with their own benefits. It also offers many defensive uses by being a steel type that resists water and is neutral to ground, while also having leftovers cancel out sr chip.
Latios also fits on all types of teams, except also dragmag in addition to sand and rain. Specs draco is the most spammable move in the tier and even though Tar is on nearly every team you can still force major damage / a kill before getting trapped. It also has a great speed tier, a spike immunity, sleep talk / trick utility, and overall great offensive checking to a lot of mons like Keld / Thund / Loom and the ability to revenge many more. Scarf is also a viable option as it outspeeds every other scarfer and dpulse is a solid cleaning move.
Lando is one of the more versatile mons here, as it can run anything ranging from scarf to fight gem to sr while also providing the general utility of intimidate. Scarf has access to the ever-spammable uturn and fight gem is nearly impossible to switch into safely.
Exca's viability is very heavily tied to it being the top spinner and spikes being so central to BW. Sand force destroys Ferro while also checking Reuni, and Mold offers a more spammable eq vs mons like Rotom. Lefties tect is amazing for scout + sand chip + its own healing but scarf has solid use too.
Chomp is really flexible with the different offensive sets it can run, whether that's orb, yache, dgem, or salac. These all can pose a serious threat due to general stab coverage combined with the variety of specifics one must account for. Scarf is also very notable due to great speed tier (scarflando, scarfdrill, volc), and outrage spammability.
Keld isn't as amazing as it was say a year ago due to Reuni limiting protect a bit but that set is still amazing vs non Reuni teams (you can trap it too). I also like the ice set that I used vs Ojama in classic because it hits dragons harder and really messes with Amoong after burn, but in general people are way more prepared for this set now. That being said, both choice sets are still scary to face in rain due to the sheer power of its water moves.
Toed is finally here because rain plays a very important role in the meta but imo it is far far worse than sand because of greater weather reliance and Toed being a worse mon than Tar in general. Protect + lefties is annoying to deal with sometimes but all other sets are really underwhelming and easy to switch into. As for rain itself, other mons in my top 20 that benefit from it / are often seen on rain are Keld, Tenta, and Thund-T.
Tran only really fits on sand (sun too) but both spdef and offensive magma sets are very good rn when you factor in protect and options like wisp / hp ice.
Glisc is essentially hazard immune, is mostly seen on sand but can fit on stuff like rain/sun stall as well. Taunt is the go to but sd is good with zone as well. Ice fang, knock, aace, and toxic are all solid utility options. It's just unkillable with protect sometimes.
Tenta is probably the sole best rain abuser in the tier because rain dish + tect offers so much recovery per turn. It's a good spinner with scald threatening water weaks / ferro and toxic / tspikes are nice utility moves.
Rotom is a mon that doesn't hard counter a ton defensively and it isn't a traditional "breaker" but it is a long term progress-maker with wisp being wisp, pump for volt stops, and volt. Even the traditionally reliable switchins like Amoong can't take sr + sand + burn + volt forever. Rotom is one of those mons you simply can't hold off forever.
Breloom has seriously improved lately with pheal rising in usage. Technician is pretty trash but pheal @ facade is so hard to switch into because it boosts and messes with every sleep absorber like Amoong and Lati can be trapped / chipped easily. Long term defensive answering requires very delicate playing around sleep usually (bar ace Glisc), otherwise you just need to revenge it with faster offensive mons. Loom is also a burn immune water res which is awesome vs scald + Rotom.
Reuni is one of the best setup sweepers in the tier because of the hazard immunity + ability to mess with other hazard immunes like Glisc with hp ice. Boltbeam + psy focus are both good options depending on what your team needs. It can still bypass Tar with focus/spikes but pursuit is an issue at the end of the day, which stops me from putting it higher.
Amoong is one of the key defensive tools for sand to not get run over by waters p much, it's also a really safe absorber of sleep considering the sleepers are Toed / Loom / other Amoong. Fire hits Ferro but ice is a solid option vs dragons and Glisc. Spore is great as always.
Zone's viability is very heavily tied to Ferro's, but balloon + mrise is also a nice option to dispose of Exca which is great atm. Zone teams can be really awk to make but as long as it traps other pokemon central to the meta it will be good.
Thund is one of the main draws of running rain, as those thunders are absurdly strong and sub/np/agility/ice/focus all let it pose a threat in different ways. 145 spa is no joke, and it outspeeds 100s + Lando Exca which is cool.
Zam has admittedly fallen out of favor recently, but sash still lets it act as an important stop to offensive mons that might otherwise get out of hand. Assuming you can keep spikes up then sash zam coming in late with the sash intact can simply checkmate your opp into the trades you want.
Rachi is pretty eh on sand but it's essential to dragmag and different sets function on rain, because it abuses thunder but also checks lati / torn nicely, and can uturn / wish as support.
Skarm closes out this list as a spiker nearly always outclassed by ferro because it's trash vs rain and way more passive in general, but helmet is still cool because it deters the omnipresent Exca spin.