Hokuto no Ken (the dub) and Guyver (again dub) brought the issue of anime to my attention. Dragonball Z and Pokémon (again, dubs) were the ones that really got me interested in anime and got me wondering what their original counterparts were like.
Speed Racer and Astroboy were the first series I've actually ever seen next to Sailor Moon.
But I didn't know what those things were. So I'd have to say Naruto was my gateway anime along with Kenshin and Inuyasha. Then I started to explore.