I don’t really spend much time on these forums anymore, but here are my opinions of the most common ones:
Fisheaters is probably the most well known of “traditional” forums, and there are a great many Catholic resources on there aside from the forum (prayers, traditional Catholic links, etc.). There was an issue some years back which I won’t discuss that caused many to leave the forum, but overall it is solid.
Suscipe Domine has more of an SSPX slant, but those who attend FSSP/ICKSP/Diocesan Latin Masses are treated well on there. I like the format of that forum and the moderators do a great job of weeding out trolls.
Cathinfo is more hard-lined SSPX, and there are many sedevacantists on there as well. I mostly stay away from reading over there because a lot of the threads devolve into uncharitable attacks on one another.
CARM is a Protestant forum that I used to frequent because they have a Catholic subforum that made for great discussion. Obviously this would not be a place to go for so-called “traditional” Catholic discussion, but it’s a great place to evangelize our non-Catholic brethren.