The answer: Arakawa Hiromu.
In my opinion, which is admittedly not a very heavy of lofty opinion, there is nothing better than having a mangaka that is able to create a real body of work. Not just having a manga that was so popular that is has crystalized a mangaka for all time, think Kubo Tite, Kishimoto, Higuchi Asa, Hatori Bisco, and Watsuki Nobuhiro when I talk about a crystalizing of an artist. Their careers have been defined by one long running epic storyline. While on one hand, this is amazing. They have entrenched themselves in the minds of readers and their respective magazines, but I have yet to see any of them write another manga.
As the great manga and anime Bakuman taught all fans, the manga world is an unforgiving place. If you can't get the editors behind a project and then reach the fans than you are cancelled and your place taken by another artist. You can have a hit and then be immortalized in the manga cannon for all time, as all of the mangaka I mentioned above are.
Yet, I can't help thinking that fact is kinda disappointing and a little sad. To only have 1 hit and 1 manga to define your career must be frustrating. When I read a book, for example, like it and connect with the world and characters that an author creates I go out and automatically try the rest of the books the author wrote. I do the same thing with manga. I've done a whole series of Adachi Mitsuru's manga and I've looked at both Tamura Yumi's epic post-apocalyptic series for that same reason. I love reading and comparing a writers/mangakas body of different series and seeing the progression of an artist because, lets be honest, that is the best part of art. Dissecting and discussing the changes and the continued fluidity of any art is some of the best arguments we can ever have.
All of us manga readers already know that Arakawa Hiromu was going to go down in manga history as the creator of one of the best shōnen manga of all time, Fullmetal Alchemist. The only anime that I am aware of that got reanimated within years of each other. No matter which anime season you watched, for the record I read the manga and then watched Brotherhood and couldn't have been more satisfied with every aspect of that anime, you knew that Ed and Al were going to become the next big thing in the manga/anime world of cosplaying. And I'm sure that everybody now knows about her next amazing manga called Silver Spoon, or Gin no Saji, that has been serialized in Shogakugan's Shōnen Sunday.