Lots of people consider Emerald the definitive Pokemon game not only on GBA, but in general, and there's very good reason for that. Hoenn is a very sensibly designed region for the most part, and Emerald effectively takes what Ruby and Sapphire had, polishes the uglier parts, and makes some good parts better while adding new story and gameplay content, while also not invalidating the other GBA games by still having version exclusives. Emerald alone perfectly defines the GBA era of Pokemon very succinctly.
I'd love to see Pokemon Emerald and other older Nintendo games get native PC ports on GOG. I would honestly even be okay with an emulated re-release as an easy way to get the ROM, as this and many other Pokemon games have becoome prohibitively expensive to find secondhand. Online battling and trading would be great as well.