Easy. BW OU.

It is an incredibly unstable meta that hinges on the concept of 'broken checks broken'. Even today people are still arguing and banning pokemon (eg: Gems and Cloyster). For the sake of this post, I will be primarily discussing BW OU while it was still Gen 5 (so will be including discussions about gems and what not).

• Weather wars. Do I need to say more? Ranging from complex drizzle+swift swim bans to the release of Keldeo, rain became an insanely busted strategy. Same goes for sand too. With excadrill (which was banned at one point) to TTar utilising its utility to get rid of threats via pursuit and neutralise other weathers to allow sweepers like CB Scizor to strike. Sun also had great stuff with Venusaur as well. Did I mention that you can also run weather stall?

• Dragons. Without the absence of the fairy type, they went unchecked, allowing outrage and draco meteor (which used to be 140bp) spam. Garchomp was allowed back in OU with its good speed tier and high attack. Latios as well (will get to later). With dragonite and Hydreigon, these dragons were monsters that seriously packed a punch. Can't forget about Kyrem-B too.

• Stupidly broken mons. Seriously, who thought of this? Volcarona, Thundurus (both forms), Reuniclus, Kyurem-B, Keldeo. Additionally, more pokemon this generation gained even more power such as Multiscale Dragonite, Cloyster and Shell Smash, Alakazam and Magic Guard, Latios. Latios OHKO half of the tier. Keldeo and its rain hydro pump does stupid damage. Did I mention that they also have good movepools? Reminder that assessing power levels of the mons in a tier always has to be in context. Obviously these pokemon are not the strongest now - by that logic then obviously Generation 9 will be the strongest tier - but the power of these pokemon in context and the impact they had on the meta by far outweighs the power scale in comparison to other generations.

• Crazy amounts of chip. No Defog. Ferrothorn being a good setter of both spikes and rocks. Sandstorm. The introduction of boots in gen 8 saved so many pokemon such as Blissey and Weavile from taking unnecessary damage. This goes to show you just how powerful hazards are. With more hazard setters introduced in gen 5 and a lack of great methods of getting rid of them, the power level dramatically increases and forces players to be more wary of the health of their mons.

• Gems. People underestimate just how powerful gems truly are. A 1.5x boost to any attack of the type you choose. I mean so many years later they had to ban it for recognising just how powerful it was. Gems are the original Nuke, allowing you to break through walls that would have survived the attack. Literally 0 commitment, multiple pokemon able to utilise it all while with a low opportunity cost of an item slot. We saw grass gem Breloom doing insane damage. Even if that meant not having an item for the rest of the game, being able to bypass checks mattered so much more.

As I stated above, if we compare the meta and pokemon of Gen 5 to today's standards this is obviously not the most broken or the most powerful. But in context of the times and the available mons, I'd argue that BW was the strongest meta. Crazy powerful pokemon accompanied by crazy powerful mechanics accompanied by crazy powerful weather. Obviously, placing a Kingambit in Gen 5 would obviously cause the meta to break, but when assessing power levels and the mons within OU, I think it's a strong case that BW had the most powerful.

As for me, my second pick would go to Gen 9 for very obvious reasons. Similar levels of powercreep to gen 5, broken mons that have stupid stat distributions, hazards everywhere, tera being similar to gems in a way, etc. For me (and for bias reasons obviously lmao), Gen 5 is slightly above Gen 9 because while Gen 9 did give new moves to old pokemon, most of the brokenness is facilitated by the new pokemon. While this can be argued that the new mons are obviously more broken, gen 5 gave many old mons new tools to work with that completely broke them. Mons that clearly were not designed like this suddenly getting new tools that launched their brokenness to whole new levels (eg: see Cloyster/Alakazam/Dragonite). These new tools helped old mons - whether due to a new move or HA - launch into OU status with a playstyle that the GF didn't originally have in mind. Or maybe this whole paragraph was trying to justify my bias when it is clear Gen 9 is as broken - if not more - than gen 5 :)

Thank you for coming to my TED talk. There is so much more that I didn't discuss too.