The only voice-synth softwares I ever used were VOCALOID 4, UTAU and openutau, and, out of these 3, my fav to use is openutau ! I absolutely despise the pitch curve drawing for tuning songs (they're ok for talkloid), since I always mess it up, especially if I'm using my mouse instead of my drawing tablet. Even though that's a feature on openutau, u can also just add extra points to the connection lines between the notes.
A thing I like more on UTAU then on openutau is how the resampler flags work, since, well, it's easier to just use the flag on a note instead of going to the settings and manually set up parameters for
every single custom flag for every single resampler I use.....
... but since UTAU is always breaking on my PC, I'll stick to openutau
Now, voice-synth softwares that I prefer to listen to, maybe VOCALOID, UTAU/openutau, Synthetizer V and
maybe NEUTRINO. I listened to some stuff using Zundamon's AI NEUTRINO voicebank and it sounded pretty good