Marshall Kirkpatrick over at ReadWriteWeb, wrote this very insightful post on identifying top blogs in any category.
Part of the conclusion is that there is no one silver bullet and thus, a tandem of free and fee approaches usually work.
For what it's worth, the answer lies in combining "automation" with the human touch. Put differently, the most valuable and/or influential blogs are not always the top 10, 25, 50 or 100 in any given category.
There's a great saying, "would you rather be head of the fox or tail of the lion", which I think works in this context i.e. pursuing the prototypical A-lister versus a hungrier and more engaged challenger blog for example.
Take a read and let me know what you think...