Raikomaru considered a hybrid by pretty much all serious growers/breeders. The owner of Barampung told me that that Raikomaru was bred in Korea and is the result of a cross between Raizan (pure) and an unnamed pink flower plant. When I asked whether the unnamed one was pure or hybrid, he said that there is no record, but he's confident that it was a hybrid.
Basically, if the color of the petals is evenly pink, it's almost guaranteed to be hybrid. Pink flowers on pure neos will be uneven or splotchy.
Benitengu is the darkest of the registered varieties. And one of the darkest of the pure species, if not the darkest. It is hard to actually determine which variety is the darkest though because growing conditions significantly affect flower color.
Also keep in mind that online photos are often misleading. Not necessarily intentionally, but because of variables in lighting, color balance, and post processing, as well as differences in display screens, what you see on the screen may or may not be what the plant looks like in real life.
Anyway, this list is by no means comprehensive but they are most of the ones shown on the Korean association profile list plus a bunch from the books that I have.
Benitengu
Shutenno
Shojo
Murasakitaishi
Togen
Kokakuden
Hien
Toko
Totenko
Rashakoryu
Tosa-Shinko
Benikomachi
Shinryuko
Benisuzume
Murasakishikibu
Kogyokunohana
Kotennohana
Shujaku
Gekkou
Tosa-Benibana
Shimane-Beni
Saihara-Beni
Shakko
Akafuji
Marumichi
Takibeni
Tosui
Shoko
Bokuen
Otoyonoaka
Minobeni
Senko
Koki
Miyabeni
Toyozakura
Ranman
Suruga-Komachi
Tokin
Tokaden
Kishubeni
Akachabo
Yachiyobeni
Shiori
Kokka
Shifuku
Taiko
Tachibanahime
Benikazusa
Benishiho
Shimanokobeni
Nishigaharabeni
Benisuo
Hisen
Benizakura
Benikocho
Shukage
Mutated Flowers
Mangekyo
Nangokunomai
Yosei
Kishu-Temari
Yatsufusa
Kutsuwamushi
Genjibotaru
Subaru
Mameba
Seikai
Hime-Seikai
Buyong
Kokuyo
Kokuryu
Gwangpung
Kogyoku
Miyahime
Momosango
Kagawarabe
Shutenno Mutations/Selections
Hanakanzashi
Oiran
Akausagi
Kokaden
Ikaruga
Shutenno-Higuma
Kayahime
Shutendoji
Rekka
Shutenno-Fukurin
Ashura
Benihanabi
Sekitei
Shutenno Shiroshima
Seohyeon
Raizan - There are some dark flower hybrid plants going around with the name Raizan, but theyre mislabeled Raikomarus. The original raizan is like the top photo in the link
Purity Uncertain
Tenkobai - Uncertain whether hybrid or pure.
Mebina - Being a seedling mutation of Tenkobai, it's likewise uncertain whether it's hybrid or pure.
Gyemyeongseong