Pokémon Crossroads will let you explore two of the franchise's best regions, Kanto and Hoenn, and you should definitely check this fan-game out.
Pokémon LeafGreen and its sibling Pokémon FireRed are as excellent today as when they released in 2004.
Pokemon FireRed and LeafGreen have two rare exclusives that are supposed to be balanced, but one is more accessible and powerful.