The difference between Nyanners' and Gura's voices is that Gura is talking near the bottom of her range (as most people do), while Nyanners is near the top. It's pretty clear when they're singing.
So basically Nyanners is high-pitched on purpose, hence sounding forced, while Gura can in fact go higher if she wants to. Her unique "voice" is probably something natural since it didn't go away when she changed characters, doesn't go away when she's singing, doesn't go away when she's stressed or surprised. For singing that's both an upside and a downside, it gives her a unique edge for sure, but it could restrict her versatility. Definitely she is quite impressive in her singing, though I think she's quite untested overall.
Suisei doesn't seem to struggle with any song in particular (that I could find). Probably puts her as second best singer in Hololive easily, she seems to hold long notes quite well. However, comparisons are quite difficult to make unless you have them all sing the same difficult thing.
As for AZKi it's worth noting that if
this profile linked by Deluxe is her, then she's been singing as a job since Gura and probably Suisei were in elementary school. I'm pretty sure that's why her singing technique is better than anyone in Hololive. Most impressively the youtube comments of her songs aren't full of doxfags, she really made a name for herself as a singer rather than a vtuber.