I actually think 20th Century Boys has a lot of problems. It's fun, but it is totally undeserved how it's put up on a pedestal. If anything, Monster is likely Urasawa's best work. I heard some insider information from someone who was a manga assistant and worked in the industry that he actually had someone else writing most of his series up until now. Odd.
i think i remember hearing it's his editor? the guy who's co-credited in the English releases of Pluto, idk though