Naoki Urasawa's Pluto is one of the best comics I've ever read, period. It's engaging on every level, doing the impossible by retelling the single most famous story from the single most famous manga creator of all time -- Astro Boy, by Osamu Tezuka -- as a murder mystery that has an incredible amount of tension and drama. On the rare occasion that anyone asks me for manga recommendations, Pluto is always at the top of my list.
That said, it's also the only Urasawa comic I've ever read. As much as I know that I should dive in for more, Monster and 20th Century Boys are two of the most prominent entries on the long list of comics that I'm sure are great but just haven't gotten around to.
When Viz announced last year that they were going to publish the complete Master Keaton, though, I decided not to let the opportunity pass me by again. After all, this was a book that sounded right up my alley; a world-traveling combination of Indiana Jones and MacGyver, and while it might not come as much of a surprise, I can assure you that the first volume is amazing.