The very first Japanese thing I ever watched was My Neighbour Totoro. I remember it being on TV at some point when I was super young, like maybe 3 or 4. The catbus is literally one of my earliest memories.
In terms of anime series that I actually followed, I think it’s Teknoman (aka Tekkaman Blade). I can’t remember the channel (this was a bit before Toonami), but I remember that they ran the penultimate episode that ended on the cliffhanger of the final battle, and then the next week was the first episode again. They literally got one episode from the end and then re-ran the whole series. My young mind was so confused. I can’t remember if I ever saw the finale.
I think then there was some Sailor Moon in there. After that, the usual Pokemon, Digimon, Dragonball Z. Tenchi Muyo. In terms of shameless toy advert animes, I remember being into Beyblade but thinking Yu Gi Oh was lame. Naruto/Bleach/One Piece were after my time.
I also read the entire Akira graphic novel series (that starts off following the movie then veers off much deeper) when I was far too young to do so, maybe 7 years old?