I'm bad at lists, so don't take my numbering as a rank of quality so much as the order they came to mind / the order I felt like talking about them:
1.) Kamijo: Bearsailles got me into Vkei and more or less took me out of my shell when it came to music and trying things that weren't on the radio or 2nd-hand listening to whatever my friends liked. His then baritone singing and melodramatic vampiric fluff had me hooked and to this day they're still one of my favorites. Granted his technique changed over time and his voice sounds a lot more natural than it did then, he's still one of my favorite vocalists and I still get a massive hard-on for all of his tacky faux-french shit. It took me a while to really like Lareine though because of how different he sounded then, but even his more effeminate style does wonders for me.
2.) Rita: E'm~grief~ is only going to be remembered by anyone who doesn't follow UCP as the band that ripped of 10 black metal bands at once, and I'm okay with that because the fact that there are still tears over this issue 4 years later perpetually amuses me. To make it better, they re-used the into of "Amentia" in their next single too, like Yuuze (the member who wrote most of their music) was saying "Hey...remember how mad you were when you heard this? " Drama aside, "Amentia" is literally the 1 odd song that doesn't mix with their discography this band put out (idk why they wanted to pretend to be black metal for a day either. Yuuze is getting old). Their "regular" songs showcased Rita's deep and bellowing vocals. In case you haven't been able to predict a trend, faux-operatic vocals are my thing. Rita is one of those no-name vocalists who do it really well, in my opinion, and are best shown in the songs "Infinity Daze" and "D.L.S." (which our favorite new drag queen is named after~). His other bands weren't the same genre of music at all. CalorZe was a fairly typical vkei band in 2005/2006 and they only had...8(?) songs.
3.) Hisui: as Hyura already mentioned, there's something so strange about his voice it's almost a miracle that he stumbled upon the genre he more or less helped to create in visual-kei. While he was emulating big names of the past, he was able to hold his own when it comes to creepy, frantic, emotionally charged vocals. While Madeth never really strayed too far away from their original style, they were able to create music to showcase how diverse Hisui could be: from mournful and out of tune, to outbursts of emotions, to sounding like he's being electrocuted: he could do it all!
4.) Hayato: DuelJewel really aren't a favorite of mine anymore. However, dude's got a pretty voice. While their discography post 2011 is more or less a joke, he's one of the few vkei guys whom have been able to pull off the operatic thing in a non-symphonic setting without coming off as a try-hard. Even their heavier songs show that what he does, he does well. Or...did.
5.) Natsuki: -OZ- kind of came out of no where with their first album. While they had been around for a while, they somehow channeled D'espairsRay and took over the scene with their first big release after a few years of making smaller works. One of vkei's biggest problems is that their growlers are awful and their heavier music is often tactless (heavy b/c we can!!!). Natsuki had the ability to switch from being aggressive to melodic while being great at both. I can see why people who weren't fans can find his voice grating, but each to his own.
6.) Klaha: More opera and gawfy dramatics. Klaha was probably one of the most stressed-out vocalists in vkei's history because he had to fill the shoes of Gackt after so much drama, a death in the band, and a complete stylistic 180. "You better work," Mana whispered into his ear upon accepting his position in the band, and he did.
7.) Yuuga: super nasally and raw, while sarcastic as fuck and probably really racist. Not a lot of people in vkei start shit because it's a social faux pas in Japan, but i guess that didn't really stop him from writing a song about cutting off oshare-kei bandmen's ears in order to ask them if they can still hear good music. He was recently resurrected after getting liposuction to sell his band's first single for 10 yen, then distribute their album for free... and apparently he's learned how to be brvtal.
Other vocalists I like but I haven't listened to their music enough to really consider them a "favorite" or I like their voice but not a lot of their band's music:
-Daisuke from Neil
-Atsushi from Buck-tick
-Yohma from Luzmelt
-(1994 and earlier) Kiyoharu
-(1989 and earlier + 1994) RYUICHI
-Shuuji 1 + Cali=/=Gari's 2nd not Shuuji vocalist (didn't they have one that like left immediately?)
I heard one of undercode's retirement contract terms basically forces them to rival other band members by entering bland pop rock projects and whoever joins the most uninspired female fronted unit rips and devours the hearts of his rivals
so far Cell wins, but this comes really close & we haven't heard that hamasaki ayumi & kisaki secret project TONExDEAFIES yet
[track list] 01.「ここで会ったが百年目!!」(「koko de atta ga hyakunenme!!」) 02.惡の秘密結社ウシノシタ団のテーマ(aku no himitsukessha ushinoshita-dan no theme) 03.改造人間「人間改造ニンゲン」(kaizou ningen「ningen kaizou ningen」) 04.デジデリオ(digi-delio) 05.いなおりベイビー(inaori baby) 06.ゴードン(gordon) 07.1999年のブルース(1999nen no blues) 08.白いテレビ(shiroi televi) 09.少女戦闘員M、踊る。(shoujo sentouin M odoru) 10.ツンドラの暴君(tsundra no boukun) 11.惡のミカタ(aku no mikata) 12.「ここは一先ず退散だ!!」(「koko wa hitomazu taisan da!!」)