Mine would definitely be Clint Langley, his artwork on some cards is simply astonishing. They really captivate the dark spirit of the Warhammer world. (Chaos Marauders, Bloodsworn, Servants of Khorne)
My honourable mentions would go to Daarken (Cover art with Broheim and Bloodthirster) and Michael Philippi (Bule, Lord of Pus and a lot of other Chaos cards).
My least favourite artist is definitely Franz Wohwinkel. Take a look at Warp Lighting Cannon. That is not a Skaven. That's Master Splinter. As far as the art is concerned, it's my least favourite card in the whole game. Otherwise it's one of my favourite ones!