While many might thing my answer would be Midnight, Mass., the comic book series I wrote for Vertigo in the early 2000s, they would be wrong. While I did create the series, and had been eager to do it for years before it finally hit stands, Midnight, Mass. went through a lot of difficulties at Vertigo before it ever hit stands, and both the initial 8-issue run and it's follow up mini-series Midnight, Mass. - Here There Be Monsters suffered from these difficulties.
My actual answer on what comic book work was most personally rewarding is twofold. On one hand I'd have to say it was Xombi a series I wrote for Milestone Media in the mid 1990s that had a 22 issue run. This series was pure me without any interference whatsoever. I had nothing but pleasurable experiences with everyone involved, particularly artist J.J. Birch, and if I could return to writing one series it would be this one. I'd love to finish what I'd planned. I suspect though That I would not be allowed the same level of freedom that I had then. My favorite issue was not issue #6, which is the issue almost everyone chooses not just as my best issue of Xombi, but my best work overall. (Issue #6 is included in the Brave and the Bold: Milestone trade paperback collection). My favorite issue was the Xombi Hanukkah Special which was illustrated by Guy Davis with color by Noelle Giddings) which was completed but never published. For the first time anywhere you can see a page from this unpublished issue below.
The second part of my answer will probably surprise most everyone. For sheer variety of satisfying stories I have to say that the work I did for the comic book incarnation of Dexter's Laboratory was probably my most satisfying work. The drawback in writing so many all ages comics, such as Dexter's Laboratory, is that no adults really read them, certainly not anyone who reviews comics professionally, nor probably anyone who edits them. The general attitude seems to be that these comics are the equivalent of Disney straight to video sequels, disposable dreck meant for an undiscerning audience. I've always found this funny in light of how seriously books like Deadpool or Gen 13, or any superhero title are taken. What that also means is that the most significant body of work to my name has gone ignored, or dismissed altogether. Unlike Scooby-Doo, there is no real formula to follow in writing Dexter's Laboratory, and pretty much anything, no matter how zany, so long as it's true to the characters and the spirit of the property, was okay to do. This in itself made it a lot of fun. The trick was that a measure of the television show's strength came from how it used beats of movement vs no-movement, reactions, and cycles of movement. It also had those vocal performances and music. These are things we didn't have, but I think we captured the spirit of the show in the most true fashion that a comic book version of it could.
Issues of Dexter's Laboratory are probably pretty cheap on ebay. To see which issues I wrote visit my comic book archives.