I rarely do straight-up redraws of their artwork to study styles, but I will occasionally go get a book off my shelf (or image-Google an artist) and see if I can't pick out a few tricks to apply to my own work.
Mike Mignola is my go-to artist when it comes to panel-compositions and page-layouts. He is a master of negative space and easily readable panels. In fact, I probably need to study him a bit more in that regard. There's also a good sense of pacing in his comics, especially the shorter ones (The Island is forever my favourite), which I could use some more study of.
Enrique Fernandez is a big inspiration when it comes to colouring. He has this great sense of lighting and atmosphere that I envy, and try to achieve in my own work.
There are a bunch of others, of course - Linda Medley for her gorgeous linework, Derib for the way he draws rocks and trees and panel-compositions, and Kawase Hasui for his highly atmospheric environments, example - but these are the first two I can think of off the top of my head.