The Most-Cited Legal Periodicals Database hosted at the Washington and Lee University Law Library measures the citations of law journals over the past 8 years (this time period was chosen in order to prevent a bias in favour of long published journals and to measure the most recent legal scholoarship). Although the methodology is not flawless, I find the information contained in this database to be interesting and instructive nonetheless.
Rankings are according to Impact Factor and Immediacy. The Impact Factor shows the average number of citations to articles in each journal (citations to a journal divided by the number of articles the journal published). The Immediacy Index is a method for comparing how rapidly the average article in a particular journal will be discovered and cited. Also look at the Ranking Methodology
Sorts are available by subject and geographic area. 40 Canadian Law Journals are ranked with the Canadian Journal of Law and Jurisprudence tied with the U of T Law Journal at the top of the Impact Factor list. There is a 4 way tie at the top of the Immediacy List. Sorting the results by an individual year is also an option.