>>69479480NTA but I'm not sure I'd call Gura's voice flexible, to me she sounds the same in nearly every song too, that soft crooner-esque club jazz voice that lends itself well to western pop as well. That doesn't bother me though, she sings beautifully, but she isn't really varying it up too much.
I can't say much about Marine, I don't really remember any of her karaokes well enough to judge how she sounds in different songs. As for Suisei, it's true that she truly shines in powerful songs and it especially lends itself well for j-rock and upbeat j-pop, but the way she sings songs like Ryuseigun and GHOST are completely different. I think she has a fantastic soft voice that she puts to great use in the former, and in covers like her Shoujo Rei.