Pokemon Emerald
Pokemon Emerald is a third edition to the Ruby and Sapphire duet. In this game, you've able to travel to many new places, like the Battle Frontier and Mirage Island. There have been many new feature changes and also there are different Pokemon missing from the in-game dex.
In this game, the designers brought back the feature that was previously installed in Crystal: Animations! The graphics are significantly better than its R/S.
Another added feature was to re-challenge Gym Leaders, who everytime, get better. Also, the Elite 4 and Gym Leaders now have new Pokemon. There's even a whole new Gym Leader and Champion!
Want to know the coolest feature of all, in my opinion? Well it's that when you get 200 Pokemon in your Hoenn Dex, you can go to Professor Birch and receive a Johto Starter!