I don't have any single answer to this. If asked in casual conversation my answer would be Yu-Gi-Oh!, but the anime that really started me down the path to otaku-dom was this old one called Idol Densetsu Eriko (or Eruka, as it's called in the (heavily-censored/'kid-friendly') Arabic dub).
Metropolis, back when I was like six years old. Back then I didn't actually know it was an anime, or what anime was. I just saw it as a cool cartoon. I've rewatched it since and I love it.