Sword Art Online. I watched the first season (completely oblivious of the existence of the next few seasons) and I just fell in love. I had a watched a few anime before it, but not many and SAO just instantly hooked me in. I loved the concept and just everything. In fact, I probably got too obsessed. I also got obsessed with Pokémon: Indigo League at the time, so these two anime were the only things I would talk about.
It was always Kirito this, Team Rocket that, and I think my friends just got SO sick of it. I have no idea how I didn't pick up on that at the time. But yeah, SAO basically consumed my life and tbh I'm grateful. If I hadn't gotten into SAO, I mightn't have gotten into anima at all!