Buffy the Vampire Slayer.
I'm not crediting the previous shows I watched as a kid because I got attached to some shows then easily. However when it comes to shows that I continued to watch season after season all the way to the end, Buffy was definitely the first television obsession I had regarding that. I started watching during its third season and I didn't stop until it ended at the seventh. I grew attached to the characters, to the story, and to the universe that was created. The show had its many ups and downs, things that I didn't like, and yet I kept with it. It's also because of Buffy that I was introduced to the online fandom in the first place, actually.
No matter what Buffy the Vampire Slayer will always be my first big fandom love, right next to Harry Potter. ♥
Masterlist of days here.