In all my visits to TO, I've always ended up checking out the ROM. The first time I went was a random suggestion to friends that resided in the city. To them it was also an unique experience since in their few years of living in the area, they never had a chance to check out the museums.
The ROM was a great experience that had many exhibitions that each provided an intense experience. One moment I'm re-educating myself about dinosaurs, the next I'm checking contemporary art from China. In honesty I haven't been to many museums to make comparisons, but the ROM will always be on my "to see" checklist whenever I visit TO.