Thats a hard one to answer. I was a 90s child who grew up on all the old Americanized shows. I think my first would be either pokemon or digimon. Thinking waaaaay back to an early saturday morning when my cousin asked which i would prefer, because they both aired at the same time.
As for like, first series in japanese. The very first thing I ever saw was an episode of M.A.R, and I remember thinking, Why is this in another language, this is weird. I closed the tab.
Years later in junior high, I was watching bleach and decided to see if I could find the next episode online. Instead i found the next 200 episodes I didn't know existed. Then I found out that the voices in Japanese, along with the dialogue and acting, was much better. Thus my eyes were opened.
My first anime was Toradora! I was like 8 at the time, so I only vaguely remember it, but I do remember it being an odd experience. It was the first show I watched that was in a foreign language so that had a lot to do with it!