I'm not sure I understand your taxonomy. In my mind, an image board is a type of forum (generally anonymous and ephemeral). Reddit is more like a forum than a typical forum than popular imageboards, since users have long-lived identities and posts are permanent by default.

As far as "most cat pictures and memes", maybe it is, by volume. But that doesn't diminish the substantial corpus of more substantive, forum-like, discussion hosted on the site.