These sometimes change but the 5 I probably follow the most are as follows (in no particular ranking):
-Brian Cook- very cartoony style, but has made a name for himself drawing butts on things.
-Alex Pardee- this guy has been around for a while and I've only recently discovered him. Very disturbing yet colorful style.
-Eric Powell- created "the goon" comic series and draws with fairly profound thick lines. Excels at drawing imperfections in people.
-Matt Inmann- the webtoonist known as "the oatmeal". Draws in basic shapes but has fairly recognizable work.
-Chris Notbusch- this one is a bit of a curve ball, as he is a sculptor not an illustrator... But his attention to detail in texturing and anatomy is pretty awesome.