My all-time favorite singer is definitely Tophamhat-Kyo, and all of RainyBlueBell by extension. Along with them, I'd say I rather enjoy Ado and Nanawo Akari... I like AKB48 too, but I don't actually know much about the individual members.
I've also been listening to Sakanamon a little recently, but I don't think I know enough to be considered an actual fan yet.
I have too many to pick, haha. Ado is definitely up there (I saw both her tours live!), but I think my all time fave is Shinji Tanimura, and his band Alice. Other contenders are every iteration of Omega Tribe, Akira Terao, and Anzen Chitai. I feel older than 29 sometimes
It feels nice to see vocaloid fans here. I like harumakigohan and iyowa
though for physical singers I listen to Phantom Siita and Atarashii Gakko!, Phantom Siita is actually produced by Ado, so if you like her, I definitely suggest checking them out!!
While the generally robotic sound can turn people off from it, I personally find it rather charming in most cases and tend to prefer that to the more realistic tuning styles... Maybe that's just some form of nostalgia bias on my part though ^^'
If you don't like the robotic sound, I suggest you maybe listen to songs that use SynthV or VoiSona, they have a more realistic sound, almost identical to an actual person. Don't force yourself to get into something you don't like, though.
There's this cover of Luna Say Maybe featuring Kasane Teto's SynthV voicebank, and it's genuinely one of the best covers I've heard. I really suggest listening to it if you like J-Rock/J-Pop and Vocaloid