Out of the ones that are out now, Ruby and Sapphire are the older ones. FireRed and LeafGreen are the newest and go back to the original Blue and Red ones from way back. My oldest DS taught himself to read by playing his "blue", so I think it is a great game. I would buy him either FireRed or LeafGreen as they are teh newest and are supposed to be real great. Both my boys are getting tehm for Christmas.
There have been many versions ~~
Red/Blue
Yellow
Gold/Silver
Crystal
Ruby/Sapphire
FireRed/LeafGreen