Ah, the RSE generation. Personally my favorite generation even though the game itself is probably the guiltiest in terms of padding game design as over half the routes were waterways meant to be traversed via Surf or Dive. This of course meant endless random encounters and trainer battles. While the game touted new additions such as abilities and double battles, my favorite thing about the RSE generation was the addition of abilities taking advantage of the changing weather conditions.
The Third Generation: Rain or Shine, Snow or Sleet
In the early stages, Game Freak tried to shake things up by introducing double battles. Trainers could choose to fight in 2-on-2 battles. The mechanics changed a bit by having Pokémon choose their targets along with other moves changing in use in doubles such as Earthquake, which would hit every active Pokémon on the field regardless of team. Doubles became a niche choice but continued to draw interest because of one other important addition Game Freak brought to RSE that made doubles interesting: abilities.
Abilities did many different things but most are a constant factor to how a Pokemon battle. Some Pokémon were supplemented by their abilities like Gyarados with Intimidate. Gyarados' defenses were somewhat ample, but Intimidate automatically lowered the opponent's attack by one stage, giving Gyarados a buffer of physical bulkiness to switch in. Other Pokémon however, we defined by their ability. Tyranitar was the sole recipient of the ability Sand Stream, which automatically summons a permanent sandstorm when Tyanitar arrived on the battlefield. Sometimes, great sounding abilities were even on unfortunate Pokémon. Sharpedo had Rough Skin, which caused damage to attackers who made physical contact with it. Unfortunately, Sharpedo was much too frail to punish physical attackers very much with its unique ability. Every so often, there would also be Pokémon that are incredible on paper but are purposely held back by their ability. Slaking is overwhelmingly powerful but Truant only allows it to move every other turn. Perhaps it's balanced by Truant?
Speaking of abilities, RSE's mascot legendaries had abilities which brought something that made weather based teams more interesting: permanent weather changes. Kyogre, the Sea Basin Pokémon, expanded the sea with its trademark ability Drizzle which would summon permanent rain until another weather condition was called in. Its nemesis, Groudon, the Continent Pokémon, evaporated water and raised land masses with its ability Drought which created permanent harsh sunlight until forcibly changed. These two abilities could of course be recreated to a lesser extent for 5 turns with their respective attacks, Rain Dance and Sunny Day.
Rain and sunny conditions enhanced certain attacks while weakening others. Water-type attacks became strong during rain while weakening fire attacks while sunlight did the opposite. More importantly, many abilities worked with special weather effects that encouraged team construction. Many rain specific teams outside of using Kyogre in the ubers tier had leads that could guarantee rain being set up on the first turn. The rest of the team would then be configured with Pokémon with the ability Swift Swim, which doubled the user's speed in rain. Good team construction dictates that the other Pokémon on the team would be enhanced by rain but would not be dependent on it. With the introduction of weather and abilities, team construction could become more diverse to function with a theme or ability rather then overall balance or tactics.
That's not to say all changes were as obvious as causing rain to suddenly come pouring down. One last big change was the addition of Pokémon natures. If you first started playing Pokémon Ruby, Sapphire, or Emerald, you may be pleasantly surprised to find out the Mudkip you were given had a Sassy personality. Well asides from assuming your Mudkip had a good sense of humor, it also meant your Mudkip excels a bit in special defense at the expense of speed. People who loved min/maxing their strategies could now catch and breed Pokémon with a specific nature to tailor them to their needs on the team. For example, you'd prefer a Zangoose that was Jolly because it sacrificed special attack, which it never uses, to increase speed so that it can get the drop on slower Pokémon. Natures are probably the biggest thing that defines whether or not a person is serious about battling other trainers as the difference in power can be quite obvious.
With the advent of strategies such as Swift Swim with rain and its opposite in Chlorophyll with sunlight, plus a much wider distribution of moves like Agility and Dragon Dance, the metagame progressed from the stall tactics from GSC and started on the rise of super offensive tactics the current generation is now known for.
As we're now over 300 Pokémon, it's become harder to point out all the big threats. Let's work our way down from returning contenders to the ones that have lost a bit of their edge, then about choices who received important abilities, and end with new threats.
Snorlax, Starmie, Blissey, and Gengar have all held firm to their position while Rhydon clings to the borderline tier while its brother in rocks Golem has since gone the way of the underused tier. Exeggutor used to see lots of usage in the previous generation but despite receiving Chlorophyll now, it's just not bulky enough to justify it's poor speed without getting sun set up. That and the existence of Blissey walling it, though to be fair, lots of Pokémon probably hate Blissey.
Tyranitar saw a huge jump in popularity when it received Sand Stream. Just as there are sun and rain teams, there exist sandstorm teams but unlike the former two weather conditions, sandstorm damages anything that's not part rock, ground, or steel-type. Dugtrio, the classic ground-type from the original RBY generation saw a resurgance when it received Arena Trap, letting it revenge kill very effectively. If your water-type got KO'ed by an electric-type, Dugtrio can come in, prevent it from switching thanks to Arena Trap, and revenge kill it with a good Earthquake. And Heracross, the plucky fighting/bug-type from Gold and Silver, also defined the strategy of absorbing status affliction moves when it received Guts this generation. Many walls like to employ the burn status to not only hurt Pokémon every turn but also neuter the attacking power of physical sweepers. But with Heracross on your team, it can take the burn and throw out extremely powerful Mega Horns thanks to Guts boosting attack by 1.5 when suffering.
Like every generation, the newcomers tend to make a splash as new threats but that also makes singling them out easier. Salamence, the new dragon-type of this generation, easily became one of the most powerful Pokémon and it just isn't still used today but it's still feared. With a diverse physical movepool, access to Dragon Dance, and Intimidate to weaken whatever it switches into, Salamence is even feared by its counters since it can take chunks out of them and once they're down, nothing else really stands much of a chance. Metagross also became popular for the obvious fact that it's part steel, which makes switching this physical brawler in a breeze. And despite stall tactics falling off in this generation, RSE also introduced Dusclops, one of Pokemon's premier stall ghost Pokémon. Not only was it tough to kill but its ability, Pressure, cause enemies to use two PP for every attack instead of one.
It goes without saying that Kyogre, Groudon, and the third legendary Rayquaza (who appropriately has an ability which negates all weather effects) were all in ubers, but it may surprise you to know that Wobbuffet was in ubers as well for its ability, Shadow Tag. Shadow Tag acts like Arena Trap except it also works on flying-types and Levitate users. It was banned to ubers because of its access to Encore, letting it trap and Encore whatever it wants, and allowing the trainer to switch in something that will appreciate the free set up time. Also, Wobbuffet vs. Wobbuffet was bemoaned because during this generation, Shadow Tag made no exception, not even to other Shadow Tag users. So if this situation occurred, it effectively became a stalemate. It has since been update to not work against other Shadow Tag users.
Weather teams were definitely powerful, but they were actually quite niche compared to other strategies. However, rain and sun did establish themselves as a powerful strategy in the lower tiers, though sandstorm is always prevalent since Tyranitar is prevalent. Like a torch bearer, Tyranitar's strength being boosted by its ability also hinted at what could happen if more Pokemon received interesting abilities, which is something we'll see in future generations. The minor addition of the hold item known as a choice band, which boosts attack by 1.5 at the cost of locking you into one move, would also act as a hint at what to expect in future generations. With Salamence and Tyranitar using Dragon Dance to boost speed and attack, the metagame is slowly snowballed into something dictated by immediate attacks and immediate results.
Next time, Pokémon finally enters the DS age! Another great split is looming in the distance and the introduction of abilities is only the beginning of what's possible! Also, like Snorlax, the introduction of one move will singlehandedly shape the metagame by itself!