In most cases, forum. Choosing the correct word requires that you fully consider and apply the analogy.
The "forum" was the city's designated market, meeting, and discussion space. In virtual space, your website is your city, and your "forum" is your designated area for meeting and discussion. (This neatly translates to a single instance of your board software.) The topics may vary, and there may be distinctions within the space for separate topics, but such divisions are expected within the greater space of the forum.
"Forums" (or, pedantically, "fora"), is the appropriate word when discussing more than one city (or website, if you are following along); it is a rare case where this is actually what a person wanted when they said "forums".
I don't know of a city that had more than one forum, thus leading to a city with "forums", but with unlimited virtual space, this consideration is unnecessary; one "forum" will suffice.