Keldeo is by far the #1 pokemon in BW if you're looking for a king, Lando as #2, everything else clumped together at #3 imo. To put it simply, those two are the only two offensive threats that are so powerful that attempting to counter them with anything but more offense is putting yourself at a disadvantage. Keld don't need any help even from SR to break things like SpD Rotom-W /Tentacruel Amoonguss in rain, and Lando isn't countered in practice by things like Blissey / Rotom-W / Gastrodon etc. (even things like Milotic if you wanted to break out the most raw bulky water possible). By in practice I mean SR and sand damage factored in, which is every game if you want it to be basically.
It almost completely limits your options for those two to those two mons we all know about. There's no such thing as running a random SpD Mew or Milo or something to tank two EPs from Lando, and there's no just using Tentacruel or Gyarados as your backup water resist for Keldeo (Gyara can't even set up on Scarf Keldeo Hydro Pump unless its basically max HP and no investment is just OHKOed by Specs Hydro after SR sometimes). If you have both of these two mons in BW2, you can't counter everything. It's said a lot but it is now the case that you won't be able to directly counter all of the top of the crop threats without relying on guesses / misses against at least one of them, which is different from the usual "this purposely built wallbreaker or uncommon set will pop up to get you once in a while".
The reason Keld is at #1 with Lando #2 is mostly because Keld wins the questionable offensive threat speed war comfortably at 108 (Lati@s aren't on the same page power wise) and Lando joins Keldeo in that tier above things like Terrakion / Garchomp / Latios etc. in terms of ability to whoop ass. All of those 3 can just be stacked against on balance or stall to ensure that it'll be something besides them that beats you, and other threats like Thund and Volc are obviously less spammable than the top two.
In case anyone wants clarity on what I mean by stacking, for any physical threat in BW2 the defensive power creep has probably outcreeped the offense. Lando-T / Quag / Tangrowth / Skarm and other physical walls will synergize with each other to the point where pure physical wallbreakers can't have an easy time against slow teams even if there's a lot of them; you have to resort to mixed stuff or different sets if you wanna try to beat stall teams (if there were any). For Latios it's sufficient IMO to just point out that Sand / Rain / Sun teams are there.
tl;dr meta is permanently offense > defense, Keldeo and Landorus are better than everything else offensive + they are the only individual pokemon that are capable of causing this shift, and Keldeo is better than Landorus.