If you'd asked me this when I was just getting into Bandori I'd have said Yukina for sure. We were in the middle of A Song Unfinished and people were picking LOUDER left, right and centre, not to mention it was playing on the event page. Yukina sounded great and LOUDER suited her voice especially well. I didn't like Kasumi's voice at all. Yes! BanG Dream implanted itself in my head and all I could think of was how shrill and grating she sounded.
Who knew she'd become my fav all these months later? Really though, whether you like Kasumi's voice or not, I think we can all agree on one thing: this girl sings her heart out. She's energetic, she's passionate and tender . . . she's so, so expressive. Yet her heightened emotions never impede the flow of her singing, only add to it (lumpy, uneven phrasing is a huge pet peeve of mine, so even if someone is more technically sound otherwise, if their singing doesn't flow well, it's still . . . eh).
You can see why Aimin had trouble with the signature Kasumi vocals in the beginning, but as she said herself (I don't remember where exactly), she eventually figured out a good way to use her voice. I'm no expert but she does sound less strained to me in more recent songs? Maybe part of it's just me getting used to how she sings but I love listening to her now.
Yukina and Kokoro are very close behind her though! It's just . . . Kasumi's range of expression, guys