Well personally I find SEO domains to be kinda annoying, and unique names to be more pleasing. That's why my topsites both have names that use the keyword "rank" for SEO, but not "topsite" or "top" at all, it would be far too easy 😛
Out of those 3, i'd have to say the first would be the easiest to rank on Google with, as it contains the most keywords.
I'd think both the first two would be equal as far as Google is concerned, since whenever I key in a plural word, I also get the singular results AND vice versa.
Maybe making a forum topsite for a set type of forums might be a better go. For example best role play forums, or best general chat forums or best gardening forums.
As loads of top sites out there are already just everything.