I've mentioned plenty of times how IRyS and Mumei are my favorites, and I basically look at it in a similar way with 'training'; IRyS is good at the pop-type stuff that is more technical singing, Mumei is good at folksy stuff, and Gura is similar to Mumei with being good at folksy stuff but is more into the jazzy stuff (which I am not as into).Agreed. Gura and Mumei for me have the best singing voices. Irys has more training and experience but if those 2 had as much experience in singing they would kill it.
IRyS and Mumei also can do a lot of other things too (I don't listen to Gura as much so I can't give examples for what else she's good at); But for IRyS you can look at the style-range easily in two of her best recent songs, Berserker with aggressive rock vocals (kinda like when she sings Jon Bovi in karaoke), and Gravity with emotional pop vocals (or just to post a live song and because I love this, Suzume, which has a hauntingly beautiful emotional vibe). And my major examples for Mumei are almost always 7 years for an emotional folksy song, and then take any pick from her songs from musicals; something like Hamilton if you want singing/rapping (she can seriously rap), or Wicked or Les Mis for a glimpse of her true power (I intentionally chose those 3 to be chronologically in the same karaoke stream).
All that said though; IRyS does blow them out of the water as far as professionalism/training (confidence (mumei pls), learning songs, making adjustments, recording, technique, etc.)
And screw it, I'll throw a shout-out to Lisa too, as a sort of 'in-between' of the pop/folk distinction that I just made up between IRyS and Mumei, because she deserves it. Stellar Stellar and Vapour Trail.