Since I read your article on Roadmap for updating our Joomla! 1.7 products to Joomla! 2.5, I have taken the task to check how much the templates have done since their birth.
I looked at the total downloads for all the templates that are still for ONLY Joomla! 1.5. A massive total of about 82 templates (there may be a little bit more as I did not find data on some of them, maybe 3 or 5 old ones).
I checked the total downloads for all of them, and wherever there was data, I ranked them accordingly - from 1 to 12 with the Highest number of downloads first. See images.
I ordered them per year, hoping this will help in finding out quickly how to prioritize. (Nothing genius). This was checked on 31st January 2012 between 15:07 to 15:30 GMT.
Most (if not all) of the 2010 and all of 2011 templates are already on Joomla! 1.7 with the latest framework, I presume. So this is based on ONLY 1.5 templates running on the old framework.
See word document attached for more details.
Joomlart Templates Downloads Stats.doc