I think I have a soft spot for those old style manga illustrations by Satoshi Shiki and Mitsuaki Sagiri with those many thin lines, but I also really like the richly detailed style of Takuhito Kusanagi for Island of the Dead God (it's not as good in Detective Duda though). Hitoshi Yoneda has become synonymous wit the Short Stories, but his artwork within the novels is often not as good as the one on the cover.

I don't really like the change in style to more chibi, uninspired, standard cliché illustrations that the series made in the late 90's / early 2000's and so I'm not a fan of Yoshikazu Hamada's style.