How about the one that used to be hosted by steemvoter where one would nominate someone else's post and the winner would be decided by the number of votes.
But since votes can be deceptive, trails and bots should be banned and to support a nomination users will rather reply to the comment with "voted", that way it's an even playing field for everyone.

You reward a winner or top 3(upto you), and probably vote on them as a reward? To encourage nominations, maybe vote on them as well?
The rewards are for you to decide though.