Mucha. Mucha. Mucha!!
*phew* Love that stuff ...
Other than that, check out Sir Lawrence Alema-Tadema. Props to Brian Froud, Graeme Base, Alex Ross, Wendy Pini ... Nearly all of my favorites are illustrators ... I do like Geiger a lot, but I tend to get the willies after looking at lots of his work in succession ... Tetsuya Nomura is my favorite game designer ever (FF7, FF8, FF10, FF10-2, FF11, FF12, Kingdrom Hearts, Parasite Eve, AND Brave Fencer Musashi--BOOYAH!) ...
And of course ... I can't forget David Mack or Terry Moore. My sequential art dieties of color and black and white respectively.