Team Fortress 2's art style is hilarious.  Cartoony without relying on overdone cellshading.  Pic related.  Also, in before hats.

Borderlands' "hand-drawn" look is cool, too.  But I also really enjoy the artistic design of Kane & Lynch 2: Dog Days.  The documentary style with compression artifacts and light washing everything out, and all of the other details they do really make for an exciting experience.
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