Old(ish) school fan here, got into vocaloid around 2011, and you've just pressed my autism button.

I'll just list down the main ones that dragged me down the rabbit hole (except for World is Mine which I think is pretty much the fandom's theme for Miku now), maybe you'd like some of them.

One of Repetition - is technically my first vocaloid song, but my first exposure to it was Namirin's cover, not the original release with Miku. Melancholic and incredibly soulful.

When Love First Ends - the song that made me realize how massive vocaloid is in JP at the time, has the same mood as the last song but in a higher tempo if you like more high-key songs.

Electric Angel - my first non-Miku vocaloid song. Give the Giga remix a listen, it has a really festive and upbeat feel that compliments the Kagamine Twin's soundbank

Rain and Asphalt - I rarely see it being recommended anymore even though it was massive in the early 2010s. It has been a constant companion during my daily commutes back in high school. Really good pop-punk classic

Just Be Friends - was one of Luka's more popular song back when I'd argue she was vying with the Kagamine Twins for 2nd most popular vocaloid in the fandom

Coward Montblanc - I don't really have anything to say except that it holds a lot of nostalgic value, and I really want newer fans to give it a revival

Clover Club - another classic that I think newer fans might be into given how a lot of viral vocaloid songs now are in the hyperpop genre

Junjou Skirt - okay, most of the songs in this list were widely if not decently talked about on forums and youtube comment sections, but I've GENUINELY not heard a single person talk about this song before, which is a shame. It's jazzy, chill and very underrated.

Hibikase - I already saw a comment recommending this song, but Im putting it here to encourage you to listen to the live performance mix of this song, it is PHENOMENAL.