Hmmm, I first listened to copycat by Circus. Found it via shitty animation memes.
Yes, it was the also very first Vocaloid song I listened to, but then i started getting really interested in who "Gumi" was and what a "Vocaloid" was.
After that, I found some lists on Top 20 Vocaloid songs or something like that and started getting more into it.
Also, Cantarella grace edition was the first song that got me into liking Kaito.
Long story short, if I hadn't been interested in that weird song from all the furry animations, I would have never joined the fandom, and also probably would have been more depressed than I am now.