I'm currently fiddling with SynthV's free version to try to write a song about werewolves -- it's not the easiest with only three tracks, but I'm chipping away. I tend to use Eleanor Forte, ANRI and Tsurumaki English -- if I weren't sticking to free stuff only then Teto might be more viable, but on the demo version she doesn't read English so flipping between voicebanks gets fiddly.
I'm a Teto fan; her resurgence seems to have reinvigorated the scene as a whole too! And of course Miku; I've been following the Miku worldwide trend on tumblr and it's been great to see. Fav Teto songs are a double billing: Song of the Eared Robot followed by Machine Love by JamieP. I'm also subbed to Mekaloton on YT (also has a spotify) who does fun electronic stuff with Teto rapping on top; it slaps. Also check out 'i guess we doin circles now' by Ex-p!