I like Alpha (Akabeesoft + some sub companies) a lot for her extraordinary sense in striking character and CG designs.
I like the KeroQ artists too, although I’m not always sure who is responsible for what, exactly. I think Motoyon is my favourite though- simply because I can see that great style in Tousui Kitan that I sure can’t see in the BLUEWATER games.
However, I will make one specific call- there’s one CG in H2O that I always found particularly striking (link, semi-NSFW) so I’ll change my favourite in this group to whoever drew that-- although a good portion of my adoration for that is in the colouring as well, but whatever.
I still think CARNELIAN does great technical work, even if her character designs are occasionally somewhat iffy. Many of the titles she’s worked on are just filled to the brim with brilliant art, although as before there’s typically one CG that comes to mind when I think of a particular artist and with CARNELIAN it’s definitely this one from Quilt - just awesome stuff, jeez.
Eroge artist KusuKusu - much like scenario writer NYAON who works with him - probably hit his peak with Moshimo Ashita ga Hare Naraba. Later titles have art that is technically good but Moshiraba’s art had FEELING, which is something I’m just not seeing in the later titles.
I also think Bekkankou is really good at drawing four different character designs over and over again. I guess the moral here is: practice makes perfect.
I probably forgot a bunch of people. There’s a lot of eroge art I really love so it’s hard to choose.
Finally, Nagaoka Hirofumi for doing things with a limited palette that I wouldn’t have thought possible. YU-NO is easily the nicest looking PC-98 game (in fact, I’d say it was the nicest looking video game ever when it came out)- there is no contest whatsoever. He only worked on that one game though 
