How well the site ranks in Google for it's target terms and title tag.
How well pages of the site rank for "keyword city"
Sites without business profiles on orphan pages.
Site design, UI and UX - Especially when showing to offline clients.
Also, sites that are indexed in Google. (I've seen services that advertise listing on some that aren't)
All of these things I listed above are more important to me than PageRank, Moz tool authority, Alexa standings etc.
I think you've seen, we keep an updated list of the top 100+ free and working sites that work for just about any U.S. business at DirectoryBug (We plan to add the top sites for a few other countries in the very near future).
Other than that list, it's good to know some main industry specific sites such as superlawyers for lawyers.
Then just do a few searches with "keywords, city" and see if any sites you may be missing are ranking on the first few pages, as well as searching "business name, phone" of the top few competitors to see if they are on any sites you may have missed.
From my experience, optimizing the profiles and making the best use of each one with customized content, keyword tags, correct categories etc. can be just as important/more important than which actual sites you create profiles.