Probably Death Note. My brother was watching it and I just sat with him without understanding wtf was going on , but then I found myself hooked in and attracted to the characters, their personalities, their motives, etc.. So I decided to watch it from the beginning, and again, and again lol. Until I realized I could spend that time discovering new anime series, so I found Hunter x Hunter (which I watched during my childhood, so it's natural I start with this one), then FMA and FMAB, and so on and so forth. I only started watching seasonally though when Attack on Titan came.
Also I've been watching Naruto during this whole period too. Even before death note, but I don't consider it my 'starting point' because I was also just sitting with my brother wasting time when I was bored.