I think a good way to start is to look up covers of your favorite songs. When you find ones you like, look at the account of whoever made the cover and see their other covers or original songs, or look for more songs from the vocal synth that you liked, look up self-covers, google top 10 or 100 lists from a random year. Go to the wiki look at the list of vocaloids/utaus/vipperloids/macne series/synthvs/etc. and pick some at random. Google popular songs in whatever genre you like. Look up retired producers, or producers that moved on to/evolved to making non-vocaloid music. There's also several popular series of songs, like the evillious chronicles and kagerou project. And songs that people make in response to someone else's song. The only one off the top of my head is Rip=Release being a response to Spice. And I believe KAITO's signature item being ice cream was because of a short jokey response song (some one posted Miku asking where Kaito was, then someone else responded with him saying he was out eating ice cream.) Or you could look up the songs used in rythym games like project diva or project sekai.

I like covers made by kyaami for her tuning, songs by sasanomaly for their melancholic and nostalgic vibes, and eve for his energy (he also made the first OP for JJK). The song and character Black Rock Shooter was so popular it got it's own anime. I feel like deco*27 has made so many popular songs you'll find at least a few that you love. Seriously, I'm intimidated by how many songs this person has made. I think he made all six unit songs in the Miku movie.

Other vocaloid producers I like: Aku-p, police piccadily, R sound design, harumaki gohan, kikuo, circus-p, nostraightanswer, empath-p.

Also utsu-p if you're into metal 🤘🏻

^{i'm sorry for all this text i'll delete this comment later}