Here are 5 that I comment on:
http://intenseblog.com -- PR5
http://comluv.com -- PR 5
http://bloggingtips.com -- PR5
http://weblogbetter.com -- PR4
http://freebloghelp.com -- PR4
You get search engine juice through a backlink from these high ranking sites in addition to other blog readers visiting that see your comments and site URL. Also these blogs use a WordPress plugin called CommentLuv where you get to have your last blog post listed with your comment.
Make sure to add a comment that contributes to the conversation, not just spam or a generic statement. Blog authors often delete comments that don't add value or can somtimes block you from commenting. If you come up with interesting or thought provoking additions to the topic you'll generate more interest in people who may also want to check your website.
Please feel free to leave comments below, also let us know of your favorite blogs for commenting.
Internet Advertising Consultant