^ Have to agree that Emerald is a great option, especially if you haven't played the Third Generation Pokemon games before. Emerald combines the best of Ruby and Sapphire, and adds a bunch of new stuff as well.
If you played the original games, then FireRed or LeafGreen are definitely worth a look, too. Not only do they have the nostalgia of following the original games' story, but new features and areas are added to it as well, giving you a little bit extra to keep you busy!