Or, in other words, what's the first Pokemon game you beat the main story in?
My first game was Diamond/Pearl, but the first time I beat the league, I was playing Platinum. The Elite Four and Cynthia were just too tough in the first two games at the time. I was relieved, and still am, that they were nerfed a bit in Platinum. They were still difficult, but not so overwhelmingly hard like they were in D/P. Eventually, I managed to win in Pearl anyway, but still.
I wish I could have played RBY and GSC back in the day, but my stepbrothers hogged them. They wouldn't share. >_>
I did get Crystal, but that was a short time before HG/SS got announced, so yeah, hardly touched it. After that, I got FR/LG and beat the league in there (Gary is such a joke... He was too easy to beat), then I beat the league AND Red in HG (and that latter battle was loooong lol). Lastly, I beat the league in Black, including Alder. He wasn't hard to beat either, though that was probably because my Reshiram was capable of trumping over half his team, and thus, made it easy. xP
I have yet to complete Ruby, Sapphire, or Emerald. I just haven't been as into them as I was the others, though I do have seven badges in Emerald so far.