Ruby, Sapphire, Emerald
#252 (Treecko) - #386 (Deoxys)
Roxanne, Brawly, Wattson, Flannery, Norman, Winona, Tate and Liza, Wallace/Juan
Elite 4 & Champion:
Sidney, Phoebe, Glacia, Drake, Steven/Wallace
Regirock, Regice, Registeel, Kyogre, Groudon, Rayquaza, Latios, Latias, Jirachi, Deoxys
Generation III has often been viewed as the awkward child of the Pokémon series. Though still incredibly popular, and certainly innovative, it was shunned by some for a less interesting world, characters and, initially, the inability to trade Pokémon from previous generations. The latter of these, at least, was remedied with the remakes of Red and Green in the form of FireRed and LeafGreen. Nevertheless, it's still a popular entry in the series and Ruby/Sapphire/Emerald are a likely target for the next remake.
Hoenn, the first region completely separate from the previous storyline, put a greater emphasis on the natural world. The three starters (Treecko, Torchic, Mudkip) sported designs closely tied to their respective elements and almost all others too, such as Lotad, Wailmer and Tropius, revolved around a more environmental theme. A total of 135 new Pokémon were added, bringing the running total to a massive 368. If you already glanced at the stats for this generation, then you may have noticed the massive increase in legendary Pokémon count, now a series standard. Gen III also happened to be the first to distribute event Pokémon wirelessly. Funnily enough, it was even the first generation that the main character had both a mother and father!
Of course, these were not the only now-standard mechanics introduced by Generation III. Abilities were introduced for the first time, so certain Pokémon, even those of the same species, had a unique aspect. Abilities such as intimidate, that could lower a foe's attack as soon as he entered combat, or the amazing drought and drizzle abilities that could bring permanent rain or sunshine respectively, brought a new edge to the competitive scene.
If you haven't caught on yet, drought and drizzle indicate another new feature to this generation: weather! Generation III brought rain and sunshine into the battle arena for the first time, which modified a number of moves that became more powerful or accurate under certain conditions. Even a new Pokémon, named Castform, was introduced that would change its appearance and stats depending on the current weather. Throw in the debut of the double battle (2 v 2) and the combat system became increasingly complex for those who wished to take advantage of the new mechanics.
Scattered amongst the larger (though arguably less well designed) world, were some new events too. Pokémon Contests, where your Pokémon could be judged on how smart, cool and beautiful they were and rewarded ribbons accordingly, made their first appearance. The Battle Tower from Crystal was expanded to include the Battle Frontier. This was a series of unique battling scenarios headed up by a special trainer, much like a Gym Leader, who had to be defeated under specific rules.
Of course, the changes didn't end there. Over 100 new moves were added, there were new gym leaders (including a change between Ruby/Sapphire and Emerald to the final gym leader and Elite 4 champion), new Pokeballs, an overhaul of berry trees and, interestingly, a removal of day and night during the game. This last change was later reintroduced in Generation IV.
Finally, and most popularly, the remakes of the original Generation I games Red and Green were released. Upgraded to FireRed and LeafGreen, the new versions brought updated graphics, new Pokémon and even new locations such as the Sevii Islands that were not present in the originals. For the time, they were real trendsetters in technology too, allowing for wireless trading between system (well, as long as you had the GBA adapter).
While I could gush endlessly about my fond memories of Pokémon Red/Blue/Yellow, as any kid growing up in the 90's understands, I want to instead give my respect to a mechanic introduced in Gen III known as Abilities. With the addition of giving every single Pokémon a passive ability, the creators managed to not only keep the franchise from growing stale, but also give some of the statistically weaker Pokémon a chance to shine.