Gilbert Shelton of the fabulous furry freak brothers. The best written of "underground comics."
Don Martin of MAD Magazine fame. If a cartoonist struggles with drawing hands and feet, they should study his work. MAD's artist were the best cartoonists of the 20th century.
Chuck Jones - Warner Brothers cartoons. Backgrounds, color and characters don't get better than this.
R Crumb I think most who hail his work do so with out really understanding it.Kind of an Andy Warhol self hype that has made him very wealthy.
Sam Gross, Gahan Wilson - Playboy magazine's staple cartoonists. Yes I flipped through the magazine to the cartoons first. My first ever rejection letter was signed by my hero Sam Gross when I submitted some gag cartoons to National Lampoon.
Berke Breathed, Bloom County. The first and only comic strip in a mainstream newspaper I ever found worth reading. I'll admit that BiNGE's look is influenced by Steve Dallas.
Terry Gilliam of Monty Python fame. Some of Python's most memorable bits are the animated cartoons from the show. Terry went on to direct many acclaimed films and demonstrate cartoonists are under rated. His films just have a completely unique look and spin on them. I wish he had built an amusement park instead of that horrible Walt Disney.
Ralph Steadman An illustrator of biblical talent. I was first exposed to his work for Hunter S. Thompson. Deeply disturbed work unmatched by others.
Matt Groening What can one say? I first was exposed to his work "Life in Hell" that ran in the Village Voice back in the 80's. Matt is the Bill Gates of cartooning. Creativity is matched by an incredible business and marketing sense. Hundreds of years from now his work could be used for an accurate account of life in 20th / 21st century America. Every social and political issue is addressed along with humanity's quirks.