It's a tough decision for me, but if I had to pick one, it would be Mike Mignola. Nobody else has quite the same style, which I really appreciate and admire. I really like the way he melds cartoony sensibilities with a gothic style.
Jack Kirby was an absolute tour de force as legendary as his works! The sheer scope of Kirby's contributions to comics, as an artist and as a writer, is truly astounding and definitively legendary: the likes of The Fantastic Four and The Avengers for Marvel and the likes of the Fourth World maxi-series and The Demon for DC are but a few choice, cherry-picked favourites! When you moreover take into consideration the sheer break-neck pace at which Kirby produced those works, it truly becomes no wonder as to why the man was rightfully crowned the King of Comics!
I also adore the magical realism that Neil Adams brought to his work, especially with regards to what he did with Batman and Deadman! (The latter moreover being his own creation!) Together, Neil Adams and Dennis O'Neal reinvigorated Batman in the wake of the silly camp that was Batman during 1950's and the 1960's. They brought Batman back to his roots as a vigilante hunting down the criminal element under the cover of a dark Gotham night, and then moreover further expanded and enriched the Bat-Lore dramatically by introducing such characters and elements as Arkham Asylum, Ra's and Talia al Ghul and their League of Assassins, and Man-Bat! In additional to all that praise for his artwork itself, I moreover have deep respect and praise for the gains Neil Adams made in the fight for creator's and artist's rights within the comics industry!
I also quite like the downright gorgeous realism Steve Dillon brought to books like Preacher. Even more impressive than Dillon's work is the even more detailed and gorgeous realism that Lee Bremejo brings to books like Joker,Noel, and Luthor!
In sharp contrast to my above praise for realism, I also like the exaggerated surrealism that artists like Sam Keith bring to their work. Such purposefully downright cartoonish artwork befits the surrealistic tone of the writing behind a book like The Maxx far more so than an attempt to produce photo-realistic panels!
I could go on and on, but those are the names that immediately leap to mind as being particularly worthy of praise!
I don't want to chime with the crowd, but I have to also say jack Kirby. He helped Stan Lee get a lot of the current Marvel characters popular. A second for me would be Todd MacFarlane. He was writing "Spider-Man" when I first got into comics as a kid. I had never seen that kind of detail before. He then switched to "The Incredible Hulk". His drawings of the Hulk remain some of the best I have ever seen.
Sean Phillips, R.M Guera, Milo Manara, Ben Templesmith, Jae lee, JRJR, Jim Lee, Darwyn Cooke, Moebius, Jacen Burrows, Dave Gibbons, Simon Bizley , Glen Fabry, Mike Mignola just to name a few off the top of my head.