In a world of hundreds of comic book writers I would have to say that my favorite would have to be Mark Millar, I mean he's written The Ultimates, The Ultimates 2, Civil War, Fantastic Four, Ultimate X-Men, Wanted, and much much more.  
 
Civlil War was amazing! Wanted was amazing! The Ultimates 1 and 2 were amazing!  Most writers aren't that consistent but Mark Millar sure is.

Millar is great.
 
Personally, I'd go Grant Morrison, Kyle&Yost, Mike Carey, Fraction (When on a solo book), Alan Moore, and Brian Michael Bendis (Again, on a Solo book.)

He is a greta writer.  
 
 
I have a couple of favorites but my all time favorite is Tom Defalco. 

my Favorites are Grant Morrison , Erik Larsen , Greg Rucka ,Mark Millar &  drum roll please ....Alan Moore ...(also Frank Miller )

My favourites are listed there. x]