The word "fora" is a piece of pretentious crap used by people who are trying to demonstrate their "intellectual superiority".
For the record, "forum" is now in the English dictionary and thus is an English word, and the correct ENGLISH plural of it is therefore "forums". This is confirmed by the OED.
"Fora" is still correct if you want to be pretentious and pretend you can speak Latin.
Ask yourself this....if you think "fora" is best, try making that your site's URL...
www.yoursite.com/fora and see how many hits you actually get.
As regards the original poll, I prefer /forum/ but that's largely because that's how Jelsoft's sites are. I did previously use /forums/ and I think both are equally ok.
(I once got so annoyed with people saying "fora" on my site that for a bit of a joke we set a replacement variable to change it to forums)