I was actually considering the idea proposed by boccaccio. The question is how to setup the system. And again, how to do the quality control. You can't just say 'good story' to become a top reviewer.
Easy! It's a discretionary award rather than based on quantity of reviews, amount of vocabulary used etc.

Just simply announce a 'reviewer of the month', at the webmaster's discretion, based on the reviewer's general contribution. No arguments!