In the last rankings, The University of Calgary took top honours and Osgoode Hall came second, with U of T 3rd, U Vic 4th and McGill 5th.
It is important to take into account the history of the school before you decide as well. Until the late 1940s, the only way to become a lawyer in Ontario was through Osgoode Hall, which was run by the Law Society of Upper Canada. The University of Toronto was not recognized by the Law Society as a degree-granting institution, so essentially a degree from U of T was the equivalent to a degree from a non-ABA approved law school in the United States. It was not until 1958 that the U of T's law program was deemed satisfactory enough for admission to the bar.
Since Osgoode Hall was the only accredited law school in Canada for so long, and was founded in 1889, it has a larger alumni (which is good for your job prospectives when you finally graduate) and a law library that has some 200,000 more volumes then the next largest legal library. It is interesting to note most of the founding partners of the large Bay Stree firms are all Osgoode graduates. Despite being the most historic, it also has the most up-to-date technology of all the law schools with wireless web in every classroom. Also, it has developed a combined LLB/JD program with NYU that will be beginning next year and is the first of its kind between a top Canadian law school with a top 5 American law school, which might be of interest to those Americans seeking both degrees.
I was accepted to every law school in Canada, and my decision came down to U Vic, Osgoode and U of T. My ultimate decision came down to the 'feel' from the open houses.
All of the law schools in Canada are high quality... unlike the United States, there isn't a law school in Canada that will accept you with a 70th percentile score on the LSAT, and so any decision you make within Canada will land you in a good law school. Everyone is biased, but I put high value on the large alumni base at Osgoode as well as the high rankings for Schulich School of Business because I am in the combined LLB/MBA program.
Well good luck making your decision!