What is the most incredible art you have ever seen in an eroge?
http://www.littlewitch.jp/home/
for me every game by Little Witch looks like high art.
The artists KusuKusu (Moshimo Ashita ga Hare Naraba) and CARNELIAN (Kao no Nai Tsuki, Touka Gettan).
So far above the rest of the competition it’s ridiculous, IMO.
I’ll add Izumi Tsubasu (Mashiroiro Symphony) to those two, although not everyone seems to be a fan of her character designs.
I don’t think I could really nominate anyone else for the absolute top tier, but there are maybe 15 or so artists I’d place on the next tier down.
Come on now ,at least link to some games they’ve worked on. 
Moshimo Ashita ga Hare Naraba, Kao no Nai Tsuki, Touka Gettan, Mashiroiro Symphony.
The next tier down would probably include artists like [listing their best/most recent works] Chikotamu (female characters in eden*), Ueda Metawo (Gun Katana, Corpse Maid), Ichikawa Saasha (Biniku no Kaori), Alpha (G-Senjou no Maou), Nekonyan (Kono Aozora ni Yakusoku wo), Fujiwara Warawara (Haruka ni Aogi, Shirokuma Bell Stars), Bekkankou (FORTUNE ARTERIAL), Kurashima Takeyasu (Ashita no Kimi to Autame ni).
I gave the names, jeez. No need to be lazy =P/
Note that with Kao no Nai Tsuki, even though the game isn’t as shiny as the other three listed, it’s worth nothing that it came out in 2000, and at the time was, without a doubt, the best looking eroge ever created.
EDIT: Oh, and I should have listed Quilt. Quilt is just beautiful. There’s a bunch of other Carnelian games and most of them look similarly good.
I’ll add to that Ueda Ryou (Soshite Ashita no Sekaiyori) and Ebina Souichi (LOVExEVOLUTION; even though he works on the worst games ever, he’s pretty clearly skilled)
Don’t forget Moekibara Fumitake (Lump of Sugar)
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and Itou Noizi (UNiSONSHIFT).
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The Lump of Sugar artist can go into the second tier, sure.
Noiji Itou goes a tier below for the occasional crimes against human eyesight, typically related to body proportions and faces. Her stuff doesn’t look bad by any means, but it doesn’t stand up to the others.
This one artist from “Light” comes to mind. Izumi Mahiru (???). I’m not sure if (s)he is a big name in the industry but I found something really striking about her art that I have yet to see in any other artist. Imitation Lover and R.U.R.U.R specifically come to mind when I think of this artist. Hasn’t done much after those games. This means I’m really excited about Soranicap because it’s been a while since i’ve seen anything noteworthy from this artist. I don’t care if it’s a nukige, or some crap game I’m gonna be all over it.
I only wish Takahiro of Tsuyokisu, Kimiaru, Majikoi fame and Mahiru could team up to make a game.
Probably Sekien no Inganock. Original and stylish.