To name some (plenty of quality European artists have been left out due to my laziness… ):
Al Feldstein did an amazing job at EC during 50's (as did Harvey Kurtzman, whose war comics are simply amazing -check out Two-Fisted Tales!)
Speaking of war comics, Archie Goodwin's Blazing Combat in 60's was very topical. Actually TOO topical, hence they forced it to stop publication.
Alan Moore is obviously the best example why British writers tend to be way superior to American ones. My favorites are V For Vendetta, League of Extraordinary Gentelemen, Lost Girls and Swamp Thing. Also wrote something called Watchmen…
Alex Toth for whatever he did. Check out Setting the Standard for the War, Romance etc he did for Standard in 50's. Also don't forget his Zorro.
Alexandro Jodorowsky is amazing visionary and had written stuff for Moebius, among others. Incal would probably be the most recommendable.
André Franquin, who did amazing stuff with Spirou et Fantasio, Gaston and marsupilami.
Bill Watterson for the amazing Calvin & Hobbes.
Brandon Graham's King City and Infinite Warhead to a lesser degree are great reads.
Brian Azzarello tend to write amazingly good yarns (100 Bullets, Hellblazer, Wonder Woman)
Brian Michael Bendis has written a LOT of stuff for Marvel. Most of it is good or at least entertaining but doesn't hold candle to his creator owned Powers.
Bruce Jones used to write (and still does) an amazing amount of quite good horror stories.
Bryan Talbot is THE master. The Advetures of Luther Arkwright is the best comic I've ever read.
Chris Claremont has written a lot of stuff, but obviously the classic X-Men material with John Byrne will always be remembered.
Cullen Bunn's Sixth Gun is an awesome western with a streak of supernatural and horror. His earlier Damned is of the same vein but takes place in the early 20th century.
Daniel Cloves' Eightball and especially Like a Velvet Glove Cast in Iron are just stunning pieces of work.
Darwyn Cooke has managed to bring sense of wonder back to comics. Check out especially his DC: New Frontier (but his Spirit and Before Watchmen stuff was als very enjoyable).
Dave Sim may be a sorry ass misogynist weirdo but especially the first two (or four) first Cerebus "phonebooks" are brilliant.
David Lapham's Stray Bullets are cool retro stories set (usually) in the 70's.
Don Lomax's Vietnam Journal is the best Vietnam comic ever. Forget 'Nam (which he also wrote some), this is the ral stuff by a real vet!
Ed Brubaker has written a ton of stuff for Marvel and it tends to be quite enjoyable, but he really shines with his creator owned material (Criminal, Fatale, Incognoto, Fade Out, Velvet) His DC material is also VERY good (Catwoman, Gotham Central, Sleeper).
Eddie Campbell's Bacchus is a delightful and clever take on the god of wine and how he is doing today. Immortality isn't forever.
Eric Shanover's Age of Bronze is an amazingly pedant but interesting and enjoyable take on Trojan War.
Evan Dorkin's Milk & Cheese is pure juvenile fun (and rage).
Frank Miller did amazing stuff with Daredevil, Batman: Dark Knight Returns and Sn City and even his later fumbles won't dilute their genious.
Garth Enis has written a lot of stuff. Usually quite brutal. Preacher is probably his greatest.
Gilbert Hernandez and Jaime Hernandez. Love & Rockets; amazingly good and diverse comic.
Georges "Herge" Remi Tintin; duh! (but also Jo, Zette and Jocko)
Grant Morrison has written an amazing amount of material. Some, if not most, of it has been written under the influence of various drugs and they are so very far out. My favorites are All-Star Superman (best Supes stories ever!), Animal Man as well as We3.
Greg Rucka wrote some of the brilliant Gotham Centrals and is currently doing brilliant work with his Lazarus. Also his Queen & Country are great takes of (British) Spy Genre.
Hayao Miyazaki for the brilliance of Nausicaa.
Hermann Huppen for especially Comanche.
Hugo Pratt. Two words: Corto Maltese.
J H Williams III is an amazingly talented artist but he and W Haden Blackman did amazing stuff with Batwoman.
Jamie Delano will always have a spot in my heart for making the early run of Hellblazer so brilliant! I especially love my old Titan B&W collections since the lack of color fits to the series' mood even better.
Jean-Michel Charlier. Les Aventures de Tanguy et Laverdure. Blueberry
Jeff Smith. Amazingly diverse writer/artist. From fantasy of Bone via Noir of RASL to pre-history of Tuki.
Jeph Loeb for the stuff he did with Tim Sale (Batman: Long Halloween and Dark Victory, etc)
Joe Hill for his amazingly intricate Locke & Key.
John Byrne for his great run with Fantastic Four.
John Wagner (& Alan Grant) for Judge Dredd.
Kazuo Koike for Lone Wolf & Cub, probably the best samurai series ever.
Katsuhiro Otomo for Akira.
Kyle Baker for his take on Plastic Man and especially on Why I Hate Saturn.
Lewis Trondheim for basically he ever did, but especially Dungeon, McConey and Mr O.
Linda Medley for her brilliant fantasy story, Castle Waiting.
Mark Millar for Ultimates, Kick Ass and Starlight (mostly his stuff is just to flex his ego it seems).
Mark Waid for his current run on Daredevil.
Matt Fraction on the brilliant duo of Hawkeye and Sex Criminals but also for Casanova and Satellite Sam.
Matt Wagner for most of the stuff he ever did but especially Grendel as well as Mage. (Also Sandman Mystery Theatre with Steven Seagle).
Milton Caniff for Terry and the Pirates and Steve Canyon
Mike Mignola for Hellboy and everything related (BPRD, Abe Sapien, Lobster Johnson, Witchfinder) as well as Baltimore-
Neil Gaiman for most of the stuff he ever wrote but, of course, mainly for Sandman and Death.
P Craig Russell for beautiful fantasy stuff (especially Elrics he did with Michael T Gilbert as well as the Oscar Wilde Fairy Tales adaptations).
Pat Mills for Nemesis the Warlock, Slaine: The Horned God and the awesome WWI comic Charlie's War.
Paul Chadwick for his eco warrrior, Concrete.
Paul Dini for the Batman & related material he did (Gotham City Sirens).
Paul Grist for Kane but to a lesser degree for Jack Staff and Burglar Bill.
Paul Pope for most of his stuff, but especially THB.
Peter Bagge for Hate!
Pierre Christin for Valerian.
Pierre "Peyo" Culliford for Smurfs, Johan et Pirlouit and Benny Breakiron.
René Goscinny for the mazing trio of Asterix, Lucky Luke and Oumpah-pah.
Rick Geary for all of his work.
Robert Kirkman for Walking Dead and Invincible.
Ross Campbell for Wet Moon.
Stan Lee (+ Jack Kirby & Steve Ditko) for creating the 60's Marvel stuff!
Stan Sakai for amazingly great samurai rabbit, Usagi Yojimbo.
Tove Jansson for the adorable Moomin.
Tsugumi Ohba for (especially the first half of) Death Note.
Wally Wood for his own stuff like Cannon and Sally Forth.
Walt Kelly for the sheer brilliance of Pogo.
Walter Simonson for his take on Thor.
Warren Ellis for amazing body of work of which plenty is highly entertaining if not brilliant. Unfortunately there are some bad misses as well. My favorites are: Desolation Jones, Planetary, Nextwave, Authority, Global Frequency and No Hero.
Wayne Vansant for many a great war comic (Days of Darkness/Wrath etc).
Wendy and Richard Pini for the riginal run of Elfquest.
Will Eisner for his graphic novels and especially for the Spirit.
Zander Cannon for his wacky Replacement God.