The Macleans ranking is usually done in November and appears in the January issue of the magazine. The 2008 rankings will be available soon. See a link in Resources below for the last rankings and be on the look out for the latest rankings. McGill University Faculty of Medicine has repeatedly ranked as the number one Canadian medical school.
Macleans' ranking is based on a survey of students and classes and faculty and of the general public to evaluate the available resources, student support and the strength of the schools' library collections. In addition since all the medical schools also have biomedical PhD programs Macleans' ranking evaluates the strength of the schools' research as well as the amount of research grants they receive.
Note that while some schools are consistently ranked top five, every medical school has what it is particularly good at or at least known for. McGill University's medical school for instance is a world-renowned research center having made recent breakthroughs in biomedical science including discovering the genes responsible for breast cancer, memory loss, spina bifida and type-2 diabetes. The University of British Columbia is known for its research centers such as the Centre for Blood Research, Centre for Disease Control and the Centre for Molecular Medicine and Therapeutics.
There has been some criticism of the way Macleans ranks Canadian universities and medical schools in particular. There is no shortage of commentary on discussion boards online for or against the ranking system and the top ranked schools. An article in the "Canadian Policy Journal" published in 2005 questioned the validity of the rankings and the statistical process used in preparing them. The article concluded that there is no evidence that highly-ranked medicals schools necessarily attracted better students or increased their enrollment share.
The American Association of Medical Colleges (AAMC), an umbrella organization that includes medicals schools in North America is not a big fan of medical school ranking either. The AAMC's stand is that all medical schools have at the core of their mission, teaching, research and patient care and since the priorities of each school differ, it is of little or no use to rank schools on just one or a few criteria.