For my vocal elite ranking, it'll be 50% skill and 50% style/preference, so it may not necessarily reflect reality that is based purely on singing skill.
1. Endou Mai - Technically her vocal is like 85 out of 100, but her intangible quality is 120 out of 100. Her singing voice is so naturally soothing and smooth that asking for anything more is insanity, she can execute ballads like as naturally as sun is the center of the solar system, however her singing style does not suit uptempo songs as well but that's a microscopic nitpicking. I'd put her in top 3 vocalists as far as female idol world is concerned.
2. Koizumi Rumi - I've had countless sleepless nights trying to sort out who is #2, and even though my encounter with Rumi is quite limited, her vocal impact leaves quite a big impression nonetheless, and while Yokoyama and Tonooka may have longer tenures as aces, I still would pick Rumi over them in terms of singing prowess. Rumi just has "it" power, when she sings on stage, it's electric, she may even top Endou in that respect, Endou still is the overall best singer, but Rumi makes up for it with powerful singing and presence. Rumi is a complete package not just as a singer, but also as an idol.
3. Yokoyama Rurika - There's really no shame in being #3 as far as Rurika is concerned, she has great range and skill to be the top vocalist in any other idol groups currently existing with ease. And since Rumi is no longer a member, technically Rurika is #2 right now. However, I'd say that the gap between Endou and Rurika is such that it would be more fitting to say Rurika is #3. Rurika is just as stable as Endou vocally and even may have better high-pitch range than Endou, but what I really don't like about Rurika's singing is that she over-sings quite a bit. Maybe the excitement of the moment gets to her too much, but she over-sings the notes by being too crafty with the lyrics in a way that it sounds odd at times. While I like Rurika's vocal tone, it's not exactly the sweet spot for my taste, so her #3 ranking is more of a reflection on my personal preference more than anything, and I wouldn't be a bit surprised if she is #1 in many other people's ranking.
4. Tonooka Erika - I really like her singing voice, it has the similar soothing feel that Endou has, but her biggest flaw is that she has no power in her vocal, and that's why she ranks below Yokoyama even though I prefer Tonooka's vocal tone much better. I'm a bit concerned that Tonooka has not improved much over time though, in fact I would say her singing has degraded recently, I wonder if she's having some kind of personal issues? Anyhow hopefully she gets rid of her inner demons and get back into form soon.
5. Tachibana Yurika - She's the undisputed ace vocal of the 3rd generation hands down, she has progressed so much so that she subs in for the center position at Shinahachi live rather frequently, and executes with great effort. I'd say her vocal range is very good, and she still has quite a bit to go before her vocal talent is fully realized. She has shown great interest in singing and also wants to learn to play guitar so her potential at this point is mostly untapped, and her vocal possibilities seem very promising. She is not as polished yet compared to 1st gen. aces, but when all is said and done, she may overtake Tonooka and rise up as the new ace because I think Yurika has more raw talent as a singer.
6. Asahi Nao - She suffers from the same thing that Tonooka does, which is lack of vocal power, plus Asahi is not as good a singer that Tonooka is, but she is just adorable on stage and her idol power makes up for it to be ranked at #6. She has improved over time as well in her singing although it's very small, but hopefully she can continue to improve and become a decent singer instead of being labeled as an idol who can't sing at all.
7. Kawamura Yui - If I go only by pure singing skill Umeko would rank in top 3 easily, but I really really do not see her as an "idol" due to her everything else aspects. I think Umeko still has untapped range that she hasn't revealed yet in an Idoling song, technically speaking she may be as good as Endou.
8. Fonchi - She is a very good singer in basis, but quite inconsistent in execution, on her good days she's as good as Tonooka, but on her bad days she's off-pitch and all over the place. She just doesn't have the technical skills to go up against the likes of Umeko. Fonchi may be a better singer than Asahi, but I consider Asahi far superior as an idol so Fonchi is ranked below.
9. Ookawa Ai - To be honest, I don't really know how good her singing is as I have rarely seen her sing solo, but at this point she seems like a decent singer who can hold down a note or two so I hope she keeps getting vocal lessons and improve gradually, especially considering that she is being pushed hard by the producer to become the next center ace.
10. Ito Yuna - I don't necessarily think she's a better singer than Gotou Kaoru, but I prefer her singing voice much better than Gotou, and she's quite raw so should have a great amount of room for improvement down the road, and quite excited about the possibilities then as a finished product.
I think I'll cut off the rankings here, because the rest are like too similar to make a clear separation in rankings really, plus there's not much nice things left to say beyond this point.