I never watched the anime but the manga was great, one of the best mystery stories out there. Though that's to be expected considering it is a Naoki Urasawa work after all.
In terms of Urasawa work though I personally would say that 20th Century Boys is a tad better than Monster. In 20th CB I cared more about the characters and even if it wasn't as psychologically complex it had a more extended plot, which I ended up preferring in the long run.