Over the years, many ranking systems for law journals have evolved, incorporating a variety of methodologies and factors, including frequency of citation, prominence of author, etc.. Although such rankings can be useful for getting an idea of the prestige or "impact factor" of a journal, they should be taken with a grain of salt and in consideration of other factors that might be important to you. Ranking of journals is frequently a subject of articles and blog postings. Play close attention to how the data was compiled---e.g. through database searches, opinions of experts in the field, etc..
Law Journal Submissions and Ranking The Washington & Lee Law School Library produces this site that lists law journals by subject, country and other factors, and allows users to rank journals by impact factor or immediacy index. (Both are based on citation counts more or less, see ranking methodology). Provides contact and submission information.
Allen Rostron & Nancy Levit, Information for Submitting Articles to Law Reviews & Journals (2019).
ISI Journal Citation Reports (Harvard ID and PIN required)
Ranks journals in a wide range of disciplines including about 100 law journals. Rankings are based on citation counts in thousands of journals in the sciences and social sciences. From the initial screen, select Social Sciences Edition and View a group of journals by Subject Category (the default). On the next screen, select Law and View journal data by either Impact Factor, Immediacy Index, or Cited Half Life.
Most Cited Journals on HeinOnline This top 100 list is based on HeinOnline's citator feature called ScholarCheck. You can also use ScholarCheck to create your own metric. They also have a collection of most-cited law journals.
Eigenfactor This is a relatively new system that ranks journals as Google ranks websites (mapping relationship structures). The coverage of law is not comprehensive, but it is useful for looking at journals in the context of the social sciences generally.
Google Scholar Metrics Google Scholar launched publication metrics in April 2012. They provide five-year h-index and h-median numbers for ranking purposes.
Measuring Quality - Writing for and Publishing in Law Reviews (Choosing Where to Submit and Publish) A great guide compiled by the Gallagher Library at the University of Washington Law School, explaining the most common ranking factors, including important an extensive selection of articles and surveys.
Is the journal available in places where scholars will find, and hopefully cite to, its contents? Some considerations include:
Is it open access or freely available? Check the journal's website for contents and the journal's policy. You can also check the Directory of Open Access Journals, but the coverage for law is not extensive.
Is it in Westlaw, Lexis and other subscription databases?
Is it indexed by Legaltrac (a.k.a Legal Resource Index)? See title list.
Is it indexed by Index to Legal Periodicals and Books? Consult journal directory. Select Index to Legal Periodicals and Books, then Display List.
Is it included in Tables of Contents Services, such as Current Index to Legal Periodicals? (See title list.)
In addition to Washington and Lee's Law Review Submissions and Ranking website, there are several directories that can be used to find out more information about law journals that are currently being published.
Many law reviews now have blogs and websites that accept shorter submissions. See Colin Miller's Submission Guide for Online Law Review Supplements, Version 7.0 and Information for Submitting to Online Law Review Companions by Bridget J. Crawford :: SSRN Washington & Lee also lists selected ones on its Law Journals: Submissions and Ranking website.