In a wordpress multisite - selfhosted environment:- how to know which blog has highest number of visitors ? how to know which blog has highest number of comments and posts ?
Google analytics can help you track down the blog visits.
This plugin may help: http://wordpress.org/extend/plugins/generalstats/
I'm working on a most comments plugin for release as well.
So far, I can;t think of any plugin off the top of my head that specifically says this blog has the most posts - though the number of posts per blog is visible in the admin area (Sites menu)