For me it was Digimon, back when the first season was on its original run. My friends were all raving about it, and being the impressionable first grader that I was, I just had to check it out. I'm glad I did, it's a fun show to watch. I guess I encountered the Pokemon anime around the same time, but that one certainly hasn't stuck with me as long as Digimon has. I don't recall how much I liked the Pokemon cartoon at the time, but I do know that I became a fan because of the games, not the show.