My first was Crystal, which I got in 2003 when I was eight along with my Game Boy Advance. (It actually came with the GBA; everyone seems very surprised when I tell them that.) Oddly, at first I actually didn't spend that much time on the game, since I couldn't even get to Violet City for an embarrassingly long time and as a result I was too frustrated to play much. (I didn't realize you were supposed to go back up the way you'd come after talking to Professor Elm the second time and the trainers battling would no longer be there; I figured there was a secret path or something I hadn't found.)
Eventually I did figure it out, and then I beat the game rather quickly and got LeafGreen, Sapphire, and Colosseum within a few months of one another.