From the past I love hyper realistic illustrations and as for today I love the work of James Jean
Gerald Scarfe, Thomas Nast, Gary Larson and M.C. Escher
Adam Hughes (he's always great on drawing women)
Marco Djurjevic (his style looks simple, but the impact, amazing)
Shinkhiro(his coloring style is very realistic, yet he didn't use model)
Shunya Yamashita (he;s also great on drawing women, in manga ways)
Sam Flores, Frank Kozik and Roy Lichtenstein are some of my favorite artists/illustrators. I'm also a big fan of a lot of comic book artists like Frank Miller, Jim Mahfood and Daniel Clowes.
I admire illustrators in the past, say NC Wyeth, Arthur Rackham, etc. There're also new coming illustrators' work catching my eyes.
There are several illustrators. I used Andrew Loomis', John Buscema's and Francis Manapul's work to learn a lot. Adam Hughes, Ed Benes, Al Rio the list goes on.
I would say that Dave Mc Kean really got me into illustration. Otherwise I'm into historic designs. I have references like "The grammar of ornements" by Owen Jones dating from mid 19th centuries or Renaissance, baroque, classical, Art deco design, etc.
Hmm, I find the work of Audrey Kawasaki and Amysol gorgeous. But I love so many out there! I don't know who to quote!
Various illustrators pre 1940's.
Hogarth & Piranesi
Oddly enough, I think my favorite has been Gil Elvgren for the longest time. But, I'm also bad at remembering names...
Frank Frazetta, is my favourite illustrator.
Past: Hokusai, Arthur Rackham, Erte, Beatrix Potter, Antonio, Edmund Dulac, Hiroshige, Hiroshi Yoshida.
Present: Hayao Miyazake, Michael Nicoll Yaguhlanaas, Lee Chul-Soo, Jean Giraud/Moebius, and about a thousand others I see online who regularly inspire me to do better work.
Past illustrators that come to mind is Alphonse Mucha and painters like Degas. Modern times I really love Michael Allred, Darwyn Cooke, Bruce Timm and any of my contemporaries I see in Imagine FX magazine.
I enjoy the following illustrators work, Alexandra Ball, Paul Holland, Madamesange, and Matthew J. Laznicka.