It took 8 chapters of Monster for me to absolutely fall in love with Naoki Urasawa’s works. So after reading Monster I just had to read 20th Century Boys and Pluto right after. Is it perfect? No. Let me address my issues with this manga.
The first problem is that it’s way too long. It’s about 100 more chapters than Monster and it just didn’t need to be.
Some of the plotlines feel like pointless meandering more than anything. One that always strikes me is the character of Koizumi. Now I like this character but she did not need to be in this manga at all and her story arc is just kinda thrown away.
20th Century Boys is different from Monster in the aspect that, while in Monster you know almost immediately who the villain is and the story is a psychological thriller, 20th Century Boys is a mystery. I’m not saying it’s a bad mystery, no it’s good, but after the main twist is revealed, one that makes me want to reread some of the earlier chapters to see how well they were setting it up, the plot just kind of lingers in a “well now what do we do” for a couple volumes.
And god, I just HATE the ending. It’s anticlimactic, feels rushed, wasn’t enjoyable anymore. The fact that the last two volumes are treated as a “sequel manga” pissed me the hell off.

Those are my gripes. Literally every other aspect is either just as good or better than Monster.

Urasawa continues to improve his art style and it looks great. There are numerous characters and almost all of them are likable, my favorite easily being Otcho. Volumes 9 and 16 are pretty much perfect. I love how information is revealed and the pace said information is revealed at. And until the end, the story was damn good as well!
I loved reading this manga. That being said, I will recommend Monster over 20th Century Boys for multiple reasons. For one, I legitimately think it’s perfect. Comparing the two, the climax/ending of 20th Century Boys feels rushed and slightly anticlimactic, while Monster dedicates itself to both its climax and ending and is so much more satisfying. It also has an anime that from what I’ve heard is a 1:1 complete adaptation, which may make it easier to get into. Don’t wait for a 20th Century Boys anime, it’d be like 120 episodes if it were 1:1. Just watch the live-action movie trilogy (which I also want to see). I know it seems like I'm underselling the great aspects of this manga but I assure you that I enjoyed reading it.

20th Century Boys is overly ambitious and couldn’t live up to what it was going for. But it’s still fantastic and worth reading.