I think we're mixing UI/UX here. User experience can be awesome with terrible UI, if the need is served brilliantly. A classified site's "experience" is in the probability of selling something, or finding it. A job portal's is similar.
I'd actually vote for FB Groups right at the top of the list. Terrible, limited UI, but the community enablement leads to great UX. Same for Team-BHP, and not too far down the list, Nextbigwhat