Pokémon Emerald is THE definitive Pokémon title for the Game Boy Advance. Like Ruby and Sapphire, the game takes place in Hoenn, but Emerald introduces several new features, such as the Battle Frontier, an improved storyline, more Pokémon available and new mechanics.
Players that own both Emerald and a copy of either Pokémon Fire Red or Leaf Green will have access to the majority of the 386 Pokémon of Generation III: most Kanto Pokémon can be found in FR/LG, most Hoenn Pokémon can be caught in Emerald, and Johto Pokémon can be found in all of them.
Moreover, for the truly hardcore Pokémon players, Emerald has 3 features that set it apart from any other Pokémon title:
-Exclusive Move Tutors: many moves are available as Move Tutors exclusively in Emerald, such as Dynamicpunch and Rollout. Even more Move Tutors can be found in the Battle Frontier, including Seismic Toss, Counter, and Softboiled. For example, Blissey can only learn Seismic Toss in Emerald or FR/LG.
-Cloning glitch: Pokémon Emerald is the only Generation III game where the player is able to clone their Pokémon. (look for guides on the internet for more information)
-Broken RNG: Emerald's Random Number Generator, or RNG, which determines your Pokémon's Stats, Natura, Ability, Gender, and even shinyness, is unlike any other. (look for guides on the internet for more information)
For these reasons and many others, Emerald is a game for every Pokémon fan.