A mixture of quality and quantity is what we're looking for, someone who posts consistently good messages regularly. Another way to think about it is ... who would you miss most from the forum if they weren't there?
That generically means the person you felt has made the best "overall contribution" rather than a specific thread. Albeit you might have a member that makes fewer posts you prefer that are all of exceptional quality. In eithercase it doesn't mean one specific thread.
You just have to weigh-it-up best you can though obviously you are going to be looking for quality posts nonetheless.