Alex Ross is a modern master. You could post almost anything from the interiors of Marvels and Kingdom Come. It's all a feast for the eyes.
If I were to break it down by decade, though, my favorites are:
1960's - Jack Kirby
1970's - Neal Adams
1980's - George Perez
1990's - Jim Lee
2000's - Alex Ross
And of these, the greatest of the greatest is still the King, Jack Kirby. Not that I consider his artwork the best, but it was years ahead of its time and the sheer amount of creativity and visual impact practically invented comics as a true artform of the 20th century. In other words, all of the ones that came after him would agree that Kirby elevated comics to heights they had never seen before him. 'Nuff said.