This is one of those things I put here some years and not others, depending on whether it occurs to me. Folding together information from the poli sci, ethics, and law Journal Citation Reports (TM) from Thomson-Reuters Web of Science, on the journals that regularly publish theory, so that it's all available in one place for theorists.
Note that Impact Factors have been shown by the Scholar General to be hazardous to your intellectual health. Use in moderation and with caution. Impact Factors can be useful to have on hand for reporting to various administrative bodies, but they are certainly overused and misused in a variety of ways. I mean to make the former easier, not to endorse the latter. This is not a substitute for a substantive assessment of quality, and this ranking has very close to no correlation with my own ranking of quality. Also note that, as always, a number of important journals including The Review of Politics and History of Political Thought are missing from these rankings altogether.
American Political Science Review: 3.688, 2nd in Political Science
American Journal of Political Science: 3.269, 4th in PS
Annual Review of Political Science: 3.140, 5th in PS
Journal of Politics: 2.255, 9th in PS
Perspectives on Politics: 2.132, 11th in PS
British Journal of Political Science: 1.987, 15th in PS
Philosophy & Public Affairs: 1.273, 40th in PS, 10th in Ethics
Political Research Quarterly: 1.149, 47th in PS
Ethics: 1.140, 13th in Ethics
International Theory: 1.051, 52nd in PS
Political Studies: 0.939, 59th in PS
Journal of Political Philosophy, 0.870, 67th in PS, 21st in Ethics
Polity, 0.642, 92nd in PS
Oxford Journal of Legal Studies, 0.641, 72nd in Law
Journal of Applied Philosophy, 0.588, 30th in Ethics
Political Theory, 0.576, 99th in PS
Law & Philosophy, 0.469, 34th in Ethics, 90th in Law
Ethics & International Affairs, 0.453, 110th in PS, 37th in Ethics
Contemporary Political Theory, 0.367, 124th in PS
Critical Review, 0.320, 130th in PS
Journal of Moral Philosophy, 0.275, 43rd in Ethics
Social Philosophy & Policy, 0.255, 44th in Ethics
Politics Philosophy & Economics, 0.214, 139th in PS, 47th in Ethics
Journal of Social Philosophy, 0.190, 49th in Ethics