Alan Moore, Pat Mills and Frank Miller remain for the time being my favorites writers since more than 20 years.
With Moore, Miller and Mills I have discovered that with a brilliant script, the art is not important (I am talking about the look of the art, not the construction/narrative aspect of the art). Back then in the late eighties I was into "pretty" drawings (I have Byrne in mind), and I had to force myself to buy French adaptation of Dark Knight Returns, Watchmen and Marshal Law because of the art, especially Kevin O'Neill's art that I found disgraceful with a lot of imperfections. Now Miller, Gibbons and O'Neill are in my top 10 artists of all time, Kevin O'Neill being my all time favourite (before Sienkiewicz!).
With Moore and Miller (but especially Moore) I have also discovered comic as a media, and what are its specific strengths (i.e. something that can be done only with this media). For me a great author is not only someone that comes with an interesting story/concept, but also someone that says "ok, here is what I want to tell, now how can I use this media the best way possible to tell it". Of what I have read recently (but I don't know much), only Joe Hill with his Locke and Key have managed in my opinion both level.
With Mills, things are more heart related. His humor, his (over the top) ideas, his universes always get me. And I deeply respect the fact that after a 40 years career, he still remembers that comic shops should be populated by kids mostly.
After them Mignola and Whedon comes to my mind. Mignola because he has became a great storyteller, and Whedon because he is in my opinion the best dialogue writer I know (and he is not bad either in term of character development and plotting).