I definitely agree with you about 20th Century Boys, this manga is my religion.
I personally enjoyed Billy Bat but I agree that it is a very very cleaving universe. I think it's one of these mangas where you can't have no opinion about it. Either you like it or not.
Yeah and I think one of the reasons for that is maybe because we read billy bat after finishing monster and 20th century boys ( at least i did )
So unconsciously we were having the mindset that billy bat will be as good or even better
And obviously it was not , which is not fair really
But even without the comparison I would maybe bump bb from 3/10 to 4/10 but nothing more
It was at times very chaotic and lacked a great main villain
honestly I donโt think thereโs any manga that can be compared to the brilliance of 20th century boys and monster
Sometimes I wish I could have a memory loss so i can reread them and experience the thrillers again