It depends on what qualifies as "watched". If you mean "what was the first anime you watched all the way through" it would be some 2012 anime like Code Geass or something. The first anime I ever remember watching seasonally was Parasyte: The Maxim, because a friend of mine told me I might like it but it turned out there was only 1 episode of it out. That's definitely when ~~the infection~~ my passion for anime began. I started watching everything I could get my hands on seasonally then. Parasyte is still a 10/10 show by the way, and it 2000% holds up today.