For me, it's definitely Senbonsakura, Or LOL (Lots of Laughs) That got me. I used to spend hours on my 2DS playing the Project Mirai DX demo, and only those two being the available songs... I so badly wanted the full game, and I did end up getting it about 3 years ago, and finally playing it...
Honesty, I grew up studio ghibli stuff (Mostly Ponyo and Spirited Away), But I think aswell as the user above me, it was for sure Sailor moon that got me hooked. The fighting, the sparkles, and the cool magical-girl outfits got my nine year-old self stunned and amazed. I still relate WAAY too...