There are actually alot of other forums out there, I think those who are more knowledgeable about this have mentioned the names and gave links to their respective websites.
But to be honest with you, Iike I read a user say in one of the comments, I also do not think that there is any other forum out there that is bigger or even half of what bitcointalk is, and I think same goes for altcoinstalks as well - will be hard or even impossible to come across any other forum that is half of what bitcointalk and altcoinstalks is respectively speaking.

I remember coming across and signing up on one crypto forum like that, I can no longer remember it's name because this was over three to four years ago, I quit visiting the forum after that first day I signed up because the forum was very boring to me, nothing important was being discussed and there were google ads scattered all over the site.