On a website, everything is tagged with keywords assigned by the staff (it's not a community driven site, due to its nature). I am able to determine which tags a user is most active in (or, what tags they view the most). However, I'm not sure how to choose the list. A few options present themselves, but they don't seem right to me.
The goal is to identify what is most interesting to the user and show them other things that they might be interested in, with respect to the tags that are assigned to the content.
A simple solution is to try several reports and check which report is more informative. The nature of your site and your data may mean that some reports are unexpectedly useful and some are not. If a report get a 'flat' area chart for example - look for something else.
Even better give the consumers of the reports a choice and an ability to provide feedback. Tune the reports based on what they will be really looking for.
P.S. I would go fro the "Take the top n tags with m views where n and m are percentages of total tags viewed and total page views" report first