I like both of these writers. They make great and entertaining comics and to me redefine what it means to be modern day writers for comic books. You may be asking why these 2? Why not Warren Ellis or Brain K Vaughn? Well, I never read their works. Warren Ellis seems like a good writer also but he tends to be of the satirizing type, which is not my thing when reading. Now I'm gonna do a breakdown of them based on these categories which are gonna be Companies and Best works/Event comics. With that said, lets begin.
Geoff is DC comics main guy. He writes for Flash, Green Lantern, Teen Titans, and others. He's responsible for bringing back Hal Jordan and Barry Allen. His best works are doing Green Lantern, I really like the way he shows the characters beyond being the space police they are meant to be. He seems to be evolving the Guardians also by making them more engaged with the events of Blackest Night.
Millar has worked with DC, but has more of an affinity to Marvel. He is responsible for creating the Ultimate Universe with the Ultimates and Ultimate X-men, and has done a few arcs with Ultimate Fantastic 4 and is now working on Ultimate Avengers. So many should thank him for creating this "interesting" universe(see previous editorial on my POV). So in terms these 2 are heavy hitters for each respectable companies.
Geoff has 4 big events on his belt. He has Infinite Crisis, 52, Sinestro Corps War and Blackest Night. These 3 events were all game changers for the company and for comic readers. In terms of stand alone comics and short runs, adventure comics 1-6 are good and His run with all things Green Lantern are a great read, and let's not forget he helped make Teen Titans cool again.
Mark Millar's only real contribution to event comics is Civil War, which helped kick off Marvels heroes to fight one another. As for his other works we have *inhales* Wanted, 1985, Old Man Logan, Wanted, Kick-ass, Nemesis, Superman: Red Son, Ultimates 1-2, his run on Sensational Spider-man, and War Heroes *tries to regain breath* Ahem. Millar knows how to make great works.
I can't wait to see what's next they decide to do in terms of their companies. Now I didn't mention status because recently Geoff Johns has become Chief Creative Officer for DC and Mark Millar is making tons of money thanks to him selling his films to studios. So there you have it, two writers that I admire in the business.