Well, I voted for Emerald.
It beats Ruby and Sapphire because it basically combines the two, and adds more (ex. all three legendaries, both teams, etc.). Plus, there's the Battle Frontier, with so much more ot do after the game.
It also beats FireRed/ Leafgreen, because...well, I liked the design of the region better, and the balance of water, forests, deserts, mountains, etc., plus the Pokemon you could get throughout the game.