I consider my first anime to be One Punch Man, I watched some mid-season episodes one night on my TV, as it was broadcasted on a channel, and I found that very cool.
Even though, I guess that technically Pokemon could be considered my first anime, or even Goldorak (my father once wanted me to watch some episodes as a child).
Later after that, I decided to begin my list of anime (with anime I really decided to watch myself) with Attack on Titan; so One Punch Man is first on the list (even if it's not an "intentional watch") and second is Attack on Titan (as the first I wanted to see).
Long message for nothing I guess ¯\_(ツ)_/¯.