What are your most successful posts?

I am trying to keep my community active. Sometimes it is difficult to get people to respond to posts - even ones that I am asking what would you like to see in the community. One of the types of posts I found is really popular are polls. Which is great, but I don't want to run a community just about polls! I would like to get more interaction. But I do think that posts that require a lot of comment writing end up dying quickly. Thoughts??